Les relations tropiques : éléments de structuration des peuplements ichtyologiques en milieu estuarien - Application à l'estuaire de la Gironde

Cette étude vise à évaluer le rôle des relations trophiques dans la structuration des peuplements ichtyologiques en milieu estuarien. Ainsi, les relations trophiques des 14 principales espèces de poissons de l’estuaire de la Gironde ont été décrites ainsi que leurs variabilités (espace, temps, taille des individus) via l’analyse des contenus stomacaux. Cette approche fait ressortir une grande diversité des types des comportements alimentaires spécifiques et une dominance de la composante temporelle dans la variabilité alimentaire, excepté pour le mulet. La méthode des analyses isotopiques du carbone et de l’azote a été utilisée ponctuellement pour évaluer les apports de cette nouvelle technique sur la connaissance de la structure et du fonctionnement des réseaux trophiques en milieu estuarien. Ce travail démontre l’impossibilité pour cet outil, de déterminer à lui seul avec exactitude les niveaux trophiques et les relations trophiques entre un poisson et ses proies, dans ce type de milieu. Il ne peut donc pas remplacer la méthode classique d’analyse des contenus stomacaux. Enfin, des guildes trophiques ont été construites à partir des résultats des analyses temporelles. Cette démarche a permis de décrire des structures trophiques saisonnières, d’émettre des hypothèses de fonctionnement et d’identifier des compartiments de proies clés. La variabilité des assemblages de poissons semble bien structurée par la dynamique saisonnière des communautés de proies, ce qui suggère un contrôle du réseau trophique plutôt de type &quote; bottom-up &quote;. L’ensemble de ce travail a permis de proposer de nouvelles guildes trophiques plus précises, applicables à l’ichtyofaune de tous les milieux d’interface.This work evaluates the role of the trophic relationships in fish assemblages as a structuring factor in an estuarine ecosystem. Thus, the trophic relationships of the 14 main fish species of the Gironde estuary are described in relation with space, time, size class, via the stomach content analysis. This approach highlighted the high diversity of specific feeding behaviours and, excepted for the mullet, a predominance of the temporal component in feeding variability. Stable isotopic analysis (carbon and nitrogen) was used to evaluate their investigative contributions to the understanding of the structure and the functioning of the estuarine trophic relationships. This work shows that this tool is not able to determine precisely the trophic levels and the relationships between a fish and its preys in such ecosystems. Thus, it can not replace the traditional method of stomach content analysis. In addition, in order to describe trophic seasonal structures, to suggest hypothesis and to identify component keys of preys, the trophic guilds were built from the temporal analysis. The variability of the fish assemblages is well structured by the seasonal dynamics of the prey communities. This observation suggests a &quote;bottom-up&quote; control of the trophic web. We finally propose a new tool enabling the description of the fish structure and the functioning of the trophic webs through accurate trophic guilds. These trophic guilds should be utilised in other transitional water ecosystems

Exploitation of trophic resources by fish under stressful estuarine conditions

Despite the high complexity and variability of estuaries, these ecosystems are very productive and play an important role in fish feeding. This paper constitutes a preliminary investigation to test how fish optimize the use of the available trophic resources, by studying trophic preference variability and feeding strategies of some pelagic and demersal fish in the Gironde estuary (southwest France). Fish and their prey were collected approximately every two months from July 2003 to June 2004 in the upstream area of the saline estuary. Stomach content analyses were realized to describe the variability of fish feeding according to their size and the time of year. Intra- and interspecific food niche overlap was evaluated using Schoener's index and a cross-calculation method was used to highlight the general fish trends in predation strategy. Stomach content results showed interspecific and intraspecific variability in fish feeding, which can be explained by their different or evolutionary ecomorphology. Their diets are composed mainly of zooplankton and hyperbenthic crustaceans with temporal variations in the consumed taxa. Optimization of the available trophic resource use, a key element in estuarine resilience, is thus possible due to the temporal adaptation of this structural trophic web. However, in spite of their temporal adaptation capacity, most fish species exhibited a specialist feeding strategy. This result was not expected, especially in the high turbidity of the Gironde estuary; the loss of one of these species could affect the fish trophic web structure and hence the resilience of the system

A Bayesian framework to objectively combine metrics when developing stressor specific multimetric indicator

In the context of the European Water Framework Directive (WFD), monitoring programs and related indicators have been developed to assess anthropogenic impacts on various components of aquatic ecosystems. While great precautions are usually taken when selecting and calculating relevant core metrics, little attention is generally paid to the generation of the multimetric indicator, i.e. the combination of the different core metrics. Indeed, most multimetric indicators are generated by simply averaging or summing metrics, without taking into account their sensitivity and their variability. Moreover, few indicators provide a rigorous estimate of the uncertainty of the assessments, while this estimation is essential for managers. In this context, we developed a Bayesian framework to build multimetric indicators aiming at improving those two weaknesses. This framework is based on two phases. First, pressure-impact statistical models are developed to quantify the impact of pressure on various fish metrics. Then the Bayesian theorem is applied to estimate probabilities of being at a certain anthropogenic pressure level from fish observation and pressure-impact models outputs. The Bayesian theorem allows to combine objectively the different core metrics, taking into account their sensitivity and their variability, and to provide rigorous uncertainty quantification, which is especially valuable in the WFD context. The method is applied as illustrative example on transitional French water bodies to demonstrate its relevance, especially in the Water Framework Directive context though the method is generic enough to be applied in various contexts

Deliverable D4.4-3, Report detailing Multimetric fish-based indices sensitivity to anthropogenic and natural pressures, and to metrics’ variation range

The Water Framework Directive (WFD) aims at achieving good ecological status (GES) for surface water bodies throughout Europe, by 2015. Consequently European countries are currently developing and intercalibrating methods based on biological, hydromorphological and physico-chemical quality elements for the assessment of their transitional waters, including fishes. The present work focuses on the response of fish indicators and indices to anthropogenic pressures and natural factors. For doing that, datasets from the Basque and Portuguese estuaries, in the North East Atlantic, have been used. Hence, biological data from fish (and in some cases, crustaceans), together with different types of pressure (population, industry, ports, dredging, global pressures, pollution, channeling, etc.) and hydromorphological data (flow, estuary volume, depth, intertidal surface, residence time, etc.) have been analyzed. Together with fish assemblages composition and individual metrics (richness, trophic composition, etc.), two fish indices (Basque AFI and Portuguese EFAI) have been investigated. Additionally, the response of five fish indices (AFI, EFAI, ELFI, TFCI, Z-EBI) were tested on a common dataset, within Portuguese estuaries, to check the time lag in the metrics’ response to different human pressures and the variability in the strength of responses to those pressures. This work also focuses on the sensitivity analysis of two European fish-based indices (French ELFI and British TFCI) to changes in their respective metric scores through their observed dynamic range. Sensitivity analyses were run simulating different scenarios of metric score changes, taking into consideration the relationship between metrics. This allowed the metrics with stronger influence in the index score and the resulting water body classification to be highlighted. Importantly, the identification of the most influential metrics could help to guide management efforts in terms of achieving GES by 2015. In general, the fish metrics and indices tested responded to anthropogenic pressures in the Atlantic estuarine sites, yet at the individual metrics level environmental chemical quality was the main driver for observed differences. Also, some metrics did not respond to pressures as expected, which is most likely related to sampling gear efficiency, namely the low capture efficiency of diadromous species with beam trawl. The cause-effect relationship study emphasized that fish-based indices developed to assess the water quality of estuarine systems did not detect all the pressures with the same sensitivity in terms of strength and time-lag, and gave more importance to some pressures, namely chemical pollution. The fish-based indices developed to assess the water quality of estuarine systems do not allow the individualization of pressure effects, which may constitute a problem to put forward the correct specific measures for management and rehabilitation of estuaries. On the other hand, some indices also do not seem relevant, in a short time, to detect changes of the ecological quality which may constitute a handicap for management or an indication for their restructuring. The sensitivity analysis indicates that a number of estuarine resident taxa, a number of estuarine-dependent marine taxa, a number of benthic invertebrate feeding taxa and a number of piscivorous taxa have the greatest influence on the TFCI classification. For the French index ELFI, the most influential metrics are mainly DT (total density) and DB (density of benthic species), followed by RT (total richness). These results suggest a high sensitivity of the quality indication provided by these indices on richness related aspects of the fish assemblages. Management should therefore prioritize efforts to conserve or restore estuarine attributes underpinning abundance and ecological diversity, for example the diversity of fish habitats, food resources and shelter or the hydrological integration between coastal and transitional waters.info:eu-repo/semantics/publishedVersio

Deliverable D4.4-5: Precision and behaviour of fish-based ecological quality metrics in relation to natural and anthropogenic pressure gradients in European estuaries and lagoons

This report summarises the work conducted in Work Package 4.4 – BQE fish in transitional (i.e. estuarine and lagoon) waters (TW) within the project WISER under the sponsorship of the European Commission. It omits most technical details of the analyses given in the four previous Work Package reports, but still provides the necessary information to understand the rationale, approach and underlying assumptions necessary to discuss the results. The focus is therefore to discuss and integrate the results obtained within Work Package 4.4 and with this, make recommendations to improve fish-based ecological assessments in TW, principally estuaries and lagoons. In addition, and to assist with the WFD implementation which is the overarching theme of WISER, the deliverable includes, where appropriate, case studies where we have used multi-metric fish indices currently under development, or already in use for WFD compliance monitoring across Europe. Furthermore, results of the work package have been shared with relevant Geographical Intercalibration Groups (GIGs) supporting the harmonization and equalization process across transitional fish indices in Europe. Development strategies for fish indices in TW vary but generally include: (1) the calibration of metrics to anthropogenic pressures, (2) the development of reference conditions, (3) the calculation of ecological quality ratios, and (4) the designation of thresholds for Ecological Status (ES) class. New fish indices are developed for a defined geographical area, using specific sampling method and under locally relevant pressure fields. The former two factors, area and sampling methods, define the relevant reference condition in the calculation of Ecological Quality Ratios (EQR) and the latter factor, human pressures, define the índex structure and especially the fish metric selection. To assess index relevance across areas, we calculated a suite of transitional fish indices on a standardized WISER dataset and then compared the agreement of the outcomes (using correlation analysis). The application of current indices to areas (or countries) different from the area in which it was originally developed leads to inconclusive or spurious results. The failure to accommodate the diferente indices to a standardized dataset in this work clearly demonstrates the fundamental reliance of current fish indices on the sampling methods and design of monitoring programmes used in the development of the index. Despite this, for some indices, correlations although weaker are statistically significant, also indicating the possible agreement in successful intercalibration between these indices. Harmonization of BQE fish methodologies across Europe (common metrics) is unlikely by adapting or creating new fish indices but inter-comparison assessments are possible and valid using a common pressure index to harmonise diferente indices on a common scale. We found a negative response of fish quality features to pressure gradients which make BQE fish in TW suitable for greater ecological integration than other BQEs. However, successful assessment of Ecological Status (ES) require a matching combination of fish index, reference values and local dataset gathered with compatible sampling methods. Whole indices provide more consistent overall ES assessments but fish metrics considered individually may be more useful as a means to focus restoration measures. Future work is needed to identify those specific pressures affecting fish assemblages providing targets for minimising the effects of stress in mitigation and restoration plans. In order to achieve this, and although the interpretation of outcomes is still difficult, more recent transitional fish indices are leading in the use of comprehensive appraisal and validation exercises to test the responsiveness of BQEs for the assessment of ES. Here we proposed for the first time a simple sensitivity exercise under realistic scenarios of metric change to explore the expected inertia (i.e. the tendency to buffer ES change after quality alterations), dynamic range (i.e. the ratio between the largest and smallest possible ES values) and most relevant metric components (i.e. the those driving the most likely scenarios leading to ES change) from a multi-metric fish índex under relevant human pressure gradients. Overall, the behaviour of multi-metric índices under manipulations of metric scores clearly indicated that metric type, number of metrics used and correlations between metrics are important in determining the index performance, with indices including more and/or uncorrelated metrics or metrics with skewed distribution being less affected by extreme metric manipulations. Results of this analysis may be used to set realistic management targets and also to identify the aspects of the indices that are more likely to affect the outcomes leading to more robust and responsive indices. Further improvements of fish indices may be attained by reducing the variability confounding biological quality metrics. This variability is undesirable noise in assessments and can be technical (i.e. linked to the method of assessment including sampling effort) or natural (physicochemical and biological). The implication for assessments is that different facts might then confound the metric-pressure correlation (the ‘signal’ in the signal-to-noise ratio in the assessments) increasing uncertainty in ES assignment. Models showed that salinity class, depth, season, time of fishing (day vs. night) and year of fishing may influence the values of the fish metrics. The modelling exercise also demonstrated that unexplained variance remains generally much higher within-systems than between-systems suggesting a higher importance of sources of variability acting at the WB level. Modelling and improved standardization in monitoring campaigns should reduce uncertainty in ES assignment. One important factor that was assessed further was the effect of sampling effort. The results suggest that richness-based metrics require larger sampling efforts although a similar effortrelated bias may be an issue for density-based metrics if fish distribution is very patchy (i.e. schooling fish or those aggregated in specific habitats) and insufficient replicates are taken to fully characterise the patchiness in their distribution. It is apparent that to overcome a potential large source of error, the Reference Conditions must be defined according to the level of effort used in the monitoring programme or, conversely, the monitoring must be carried out at the same level of effort used to derive the Reference Condition. The WP finally explored the use of a predictive linear modelling approach to define reference conditions for fish metrics in transitional waters. The fish response data was modelled together with Corine Land Cover (CLC)-derived pressure proxies (% agricultural, urban and natural land coverage). Based on the obtained models, the expected metric score was predicted by setting pressure levels either to the lowest observed pressure in the dataset or to zero in order to define the sample and theoretical reference condition, respectively. Even when significant, the effect of pressures on fish metrics was generally very weak, probably reflecting the use of too-generic pressure indicators (such as land cover data instead of more relevant estuarine proxies such as dredging, port development, waterborne pollutants, etc). The best explanatory models included sampling factors and natural characteristics considered important discriminant features in the definition of water body types. In particular, the present work argues for considering not only estuaries and lagoons as different typologies but also other natural and design characteristic such as the gear type, the sampling season and the salinity class. Furthermore, a relevant reference needs to account for survey design bias, including rare species contribution to assessment datasets, patchiness, choice of pressure proxies or sampling gear. The modelling approach of fish metrics against the physicochemical variables has proved useful to derive Reference Conditions. This is important for the computation of relevant EQRs in Europe where there is a general lack of pristine areas or historical data on fish BQE and it provides an alternative to best professional judgment. Taking all WP analysis and case studies together, the work conducted has highlighted the following key messages and linked research needs necessary to optimize BQE fish for the quality assessment of transitional waters: Key Message 01: Harmonization of BQE fish methodologies across Europe (common metrics) is unlikely by adapting or creating new fish indices but inter-comparison assessments are possible and valid using a common pressure index to harmonise diferente indices on a common scale. Research needs to be focused on more widely-applicable fish indices will require the formulation of completely new indices based on a more flexible use of fish metrics according to system typologies, relevance and, probably, an increased use of functional traits. For current indices, further research on a method of intercalibration is needed. Key Message 02: BQE Fish in TW respond consistently to human pressure gradients across transitional waters providing the means to assess Ecological Status (ES). Further work will be needed to identify those specific pressures affecting fish assemblages providing targets for minimising the effects of stress in mitigation and restoration plans. Key Message 03 Although the interpretation of outcomes is still difficult, more recente transitional fish indices are leading in the use of comprehensive appraisal and validation exercises to test the performance of BQEs in the assessment of Ecological Status (ES). Further appraisal of fish indices behaviour is needed to understand the meaning of the quality outcomes, to set realistic management targets and also to identify the aspects of the índices that are more likely to affect the outcomes leading to more robust and responsive indices Key Message 04 Uncertainty levels associated with metric variability in multi-metric fish indices can be managed to increase the confidence in Ecological Status (ES) class assignment. Further research is needed to include knowledge of habitat partition within systems, to understand metrics behaviour and precision, to test new combination rules allowing metric weighting by robustness and importantly to evaluate more robust sampling tools and methods. Key Message 05 Reference conditions for BQE fish-based quality assessments can be objectively estimated using predictive modelling. Further refinements will require the use of better pressure proxies, robust metrics amenable to modelling and to account for survey design bias (effort & choice of sampling gear) at the relevant scales used in monitoring programmes.info:eu-repo/semantics/publishedVersio

A critical look at the definition of indicators to assess the effectiveness of marine protected areas

Marine protected areas (MPA) have been created to protect threatened, rare species and/or preserve remarkable habitats. What are the results, have they succeeded in restoring biodiversity? Are the selected indicators suitable for the task? How can they be improved

Regard critique sur la mise en place d’indicateurs d’évaluation de l’efficacité des aires marines protégées

Les aires marines protégées (AMP) ont été créées pour protéger des espèces rares menacées et/ou préserver des habitats remarquables. Quel est le bilan de ces espaces ? Ont-ils permis de restaurer la biodiversité que l'on a souhaité conserver ? Le choix des indicateurs est-il toujours pertinent ? Comment les améliorer

Recherches sur les réseaux trophiques en milieu estuarien

National audienceEstuarine areas are generally submitted to important human pressure. In order to maintain and/or restore the quality of estuarine ecosystems, it is necessary to describe their structure and functioning. In that way, many recent scientific works focus on food webs, which are depicted as being good indicators of the functioning of aquatic ecosystems. But how can estuarine food webs be described? This paper proposes a pragmatic and practical review of the most widely used techniques (stomach/gut content analysis, stable isotopes ratios and biochemical markers) with emphasis on their main advantages, drawbacks and bias according to possible goals to reach for estuarine ecologist. Even if these approaches are different, they provide complementary information about the trophic relationships between the components of the system, that is to say the sources of organic matter and the description of energy flows between the different compartments of the food web.. All these results can be synthesized with the use of trophic models to provide a picture of the global estuarine trophic structures which is an essential step towards the comprehension of the functioning of these ecosystems

Les relations trophiques : éléments de structuration des peuplements ichtyologiques en milieu estuarien. Application à l'estuaire de la Gironde

This work evaluates the role of the trophic relationships in fish assemblages as a structuring factor in an estuarine ecosystem. Thus, the trophic relationships of the 14 main fish species of the Gironde estuary are described in relation with space, time, size class, via the stomach content analysis. This approach highlighted the high diversity of specific feeding behaviours and, excepted for the mullet, a predominance of the temporal component in feeding variability. Stable isotopic analysis (carbon and nitrogen) was used to evaluate their investigative contributions to the understanding of the structure and the functioning of the estuarine trophic relationships. This work shows that this tool is not able to determine precisely the trophic levels and the relationships between a fish and its preys in such ecosystems. Thus, it can not replace the traditional method of stomach content analysis. In addition, in order to describe trophic seasonal structures, to suggest hypothesis and to identify component keys of preys, the trophic guilds were built from the temporal analysis. The variability of the fish assemblages is well structured by the seasonal dynamics of the prey communities. This observation suggests a "bottom-up" control of the trophic web. We finally propose a new tool enabling the description of the fish structure and the functioning of the trophic webs through accurate trophic guilds. These trophic guilds should be utilised in other transitional water ecosystems.Cette étude vise à évaluer le rôle des relations trophiques dans la structuration des peuplements ichtyologiques en milieu estuarien. Ainsi, les relations trophiques des 14 principales espèces de poisson de l'estuaire de la Gironde ont été décrites ainsi que leur variabilité (espace, temps, taille des individus) via l'analyse des contenus stomacaux. Cette approche fait ressortir une grande diversité des types de comportements alimentaires spécifiques et une dominance de la composante temporelle dans la variabilité alimentaire, excepté pour le mulet. La méthode des analyses isotopiques du carbone et de l'azote a été utilisée ponctuellement pour évaluer les apports de cette nouvelle technique sur la connaissance de la structure et du fonctionnement des réseaux trophiques en milieu estuarien. Ce travail démontre l'impossibilité pour cet outil, de déterminer à lui seul avec exactitude les niveaux trophiques et les relations trophiques entre un poisson et ses proies dans ce type de milieu. Il ne peut donc pas remplacer la méthode classique d'analyse des contenus stomacaux. Enfin, des guides trophiques ont été construites à partir des résultats des analyses temporelles. Cette démarche permet de décrire des structures trophiques saisonnières, d'émettre des hypothèses de fonctionnement et d'identifier des compartiments de proies clés. La variabilité des assemblages de poisson semble bien structurée par la dynamique saisonnière des communautés de proies, ce qui suggère un contrôle du réseau trophique plutôt de type "bottom-up". L'ensemble de ce travail a permis de proposer de nouvelles guildes trophiques plus précises, applicables à l'ichtyofaune de tous les milieux d'interface

Utilisation des guildes trophiques ichtyologiques pour décrire la structure et le fonctionnement des systèmes d'interface

International audienceThe functioning of the Gironde estuary (France) has been explored using fish trophic guilds, i.e. groups of species that exploit similar resources within a community. These trophic guilds have been built from the feeding description of the 14 most frequent fish species, collected seasonally between July 2003 to April 2004. Generally, four major trophic guilds have been identified. Among them, crustacean and zooplankton predators were dominated. The absence of the piscivore guild confirmed the protection function for fish of estuarine ecosystems. The seasonal analysis of the fish trophic guilds reflected an high seasonal variability of estuarine functioning: - in summer, the fish assemblages were essentially supported by the zooplanktonic compartment - the richness of the autumnal and spring fish assemblages reflected an high diversity in preys communities, determining a bentho/pelagos coupling - and in winter, the low fish species richness seemed to result from a trophic capacity limited. These observations suggested a bottom-up control of the estuarine food web. But beyond the understanding of the trophic dynamics, the guild concept proved to be a useful tool to detect anomalies of functioning such as a strong competition pressure exerted by the meagre (its high abundance having been linked to the global climatic change) suggesting a top-down effect and threatening the stability/resilience of the system. A comparative approach of different interface systems, will be very useful for monitoring and managing these complex ecosystems.Devant la nécessité d'étudier la structure et le fonctionnement des milieux d'interface (estuaires, lagunes, baies semi-fermées ), certains auteurs se sont attachés à diviser les communautés de poissons en guildes trophiques, c'est-à-dire en groupes d'espèces présentant des similitudes alimentaires. Cette démarche a été adoptée dans l'estuaire de la Gironde. Les guildes trophiques ont été construites à partir de la description des régimes alimentaires des 14 principales espèces de poissons, récoltés saisonnièrement entre juillet 2003 et avril 2004. Globalement, quatre guildes trophiques majeures ont été identifiées. Parmi elles, les guildes prédateurs de crustacés et de zooplancton dominaient. L'absence de la guilde des piscivores a confirmé la fonction de protection attribuée aux écosystèmes estuariens vis à vis de l'ichtyofaune. Les analyses saisonnières des guildes trophiques des poissons reflètaient une forte variabilité saisonnière du fonctionnement estuarien : - en été, les assemblages de poissons étaient essentiellement supportés par le compartiment zooplanctonique la richesse automnale et printanière des assemblages de poissons reflétaient une forte diversité dans les communautés de proies, déterminant un couplage benthos/pelagos en hiver, la faible richesse spécifique des poissons semblait résulter d'une capacité trophique limitée. Ces observations suggéraient un contrôle du réseau trophique estuarien de type bottom-up. Mais, au-delà de la compréhension des dynamiques trophiques, le concept de guilde s'est avéré être un outil utile pour détecter des anomalies de fonctionnement, telle qu'une forte pression de compétition exercée par le maigre, Argyrosomus. regius (sa forte abondance ayant été reliée au changement climatique global), suggérant un contrôle du système de type top-down, menaçant la stabilité/résilience du système. Une approche comparative des différents milieux d'interface, utilisant la même méthodologie serait très utile pour surveiller et gérer ces écosystèmes complexes