Résumé: End-to-end ecosystem modeling platforms, including OSMOSE, are key tools for informing ecosystem-based fisheries management (EBFM). End-to-end models ideally implement two-way interactions between model components, yet two-way interactions between high trophic level (HTL) functional groups and humans (fisheries managers and fishers) are currently missing in OSMOSE. We developed a management strategy evaluation (MSE) framework for OSMOSE, which allows for feedback between HTL functional groups and fisheries managers. This framework couples OSMOSE to a management procedure integrating decision rules and accounting for scientific uncertainty and the acceptable risk of overfishing. We applied the MSE framework to the OSMOSE model of the West Florida Shelf, so as to conduct an evaluation of total allowable catch (TAC) strategies for red grouper (Epinephelus morio) in a context of episodic events of natural mortality. Our simulations indicate that TAC strategies that assume higher scientific uncertainty and/or lower acceptable risk of overfishing result in higher biomass-related metrics for red grouper. However, the levels of scientific uncertainty and acceptable risk of overfishing impose a trade-off between biomass-related and catch-related metrics for red grouper. Our simulations also indicate that updating red grouper TAC more frequently in a context of episodic events of natural mortality does not have a large impact on biomass-related and catch-related metrics for red grouper and other functional groups. The MSE we conducted for red grouper is strategic, and its outcomes, which were obtained under a specific set of assumptions, must be considered preliminary. We discuss how future research could help enhance understanding of the possible impacts of TAC strategies for red grouper. The MSE framework designed for OSMOSE links the dynamics of HTL functional groups to that of fisheries managers, thereby allowing OSMOSE to be better suited for informing EBFM. This framework is an invaluable asset in assessing the performance of fisheries management strategies, but could also be used for other purposes, such as the evaluation of research monitoring programs.

Résumé: Given the ecological importance and high socio-economic value of the fishery of the Gulf of Gabes, an end-to-end model was applied to its continental shelf ecosystem to characterize the structure of the food web in the 2000s. This approach consisted in forcing a high trophic level model (OSMOSE) with an existing biogeochemical model (Eco3M-MED) representing the seasonal dynamics of the low trophic levels. The two models were linked through trophic interactions to represent the ecosystem dynamics from primary producers to top predators. In this study, we developed the multispecies, individual-based model OSMOSE in the Gulf of Gabes (OSMOSE-GoG). This model aims to capture the main processes that influence species life cycle and simulate the functioning of the ecosystem according to opportunistic predation process based on size selection and spatio-temporal co-occurrence between a predator and its prey. The spatial distribution of the eleven modelled species was derived from a Multi-Scale Species Distribution Modelling approach. We calibrated OSMOSE-GoG model with available data of biomass and fishing yield, using an optimization method based on evolutionary algorithms which is suitable for complex and stochastic models. Finally, OSMOSE-GoG was validated against independent data sets at different hierarchical levels: the individual (diet composition), population (mean size of commercial catch) and community levels (mean trophic level) following the Pattern-Oriented Modelling approach. The model outputs were overall consistent with the diet compositions and mean trophic levels derived from the ECOPATH model of the Gulf of Gabes (ECOPATH-GoG) and the observations of mean size of catches. The OSMOSE-GoG can be considered as a baseline model to investigate ecosystem responses to environmental changes and fishing management measures in the Gulf of Gabes.

Résumé: Understanding how external pressures impact ecosystem structure and functioning is essential for ecosystem-based approaches to fisheries management. We quantified the relative effects of fisheries exploitation and environmental conditions on ecological indicators derived from two different data sources, fisheries catch data (catch-based) and fisheries independent survey data (survey-based) for 12 marine ecosystems using a partial least squares path modeling approach (PLS-PM). We linked these ecological indicators to the total biomass of the ecosystem. Although the effects of exploitation and environmental conditions differed across the ecosystems, some general results can be drawn from the comparative approach. Interestingly, the PLS-PM analyses showed that survey-based indicators were less tightly associated with each other than the catch-based ones. The analyses also showed that the effects of environmental conditions on the ecological indicators were predominantly significant, and tended to be negative, suggesting that in the recent period, indicators accounted for changes in environmental conditions and the changes were more likely to be adverse. Total biomass was associated with fisheries exploitation and environmental conditions; however its association with the ecological indicators was weak across the ecosystems. Knowledge of the relative influence of exploitation and environmental pressures on the dynamics within exploited ecosystems will help us to move towards ecosystem-based approaches to fisheries management. PLS-PM proved to be a useful approach to quantify the relative effects of fisheries exploitation and environmental conditions and suggest it could be used more widely in fisheries oceanography.

Résumé: Fisheries provide critical provisioning services, especially given increasing human population. Understanding where marine communities are declining provides an indication of ecosystems of concern and highlights potential conflicts between seafood provisioning from wild fisheries and other ecosystem services. Here we use the nonparametric statistic, Kendall׳s tau, to assess trends in biomass of exploited marine species across a range of ecosystems. The proportion of ‘Non-Declining Exploited Species’ (NDES) is compared among ecosystems and to three community-level indicators that provide a gauge of the ability of a marine ecosystem to function both in provisioning and as a regulating service: survey-based mean trophic level, proportion of predatory fish, and mean life span. In some ecosystems, NDES corresponds to states and temporal trajectories of the community indicators, indicating deteriorating conditions in both the exploited community and in the overall community. However differences illustrate the necessity of using multiple ecological indicators to reflect the state of the ecosystem. For each ecosystem, we discuss patterns in NDES with respect to the community-level indicators and present results in the context of ecosystem-specific drivers. We conclude that using NDES requires context-specific supporting information in order to provide guidance within a management framework.

Résumé: ABSTRACT: Trophic level (TL)-based indicators have been widely used to examine fishing impacts in aquatic ecosystems and the induced biodiversity changes. However, much debate has ensued regarding discrepancies and challenges arising from the use of landings data from commercial fisheries to calculate TL indicators. Subsequent studies have started to examine survey-based and model-based indicators. In this paper, we undertake an extensive evaluation of a variety of TL indicators across 9 well-studied marine ecosystems by making use of model- as well as survey- and catch-based TL indicators. Using detailed regional information and data on fishing history, fishing intensity, and environmental conditions, we evaluate how well TL indicators are capturing fishing effects at the community level of marine ecosystems. Our results highlight that the differences observed between TL indicator values and trends is dependent on the data source and the TL cut-off point used in the calculations and is not attributable to an intrinsic problem with TL-based indicators. All 3 data sources provide useful information about the structural changes in the ecosystem as a result of fishing, but our results indicate that only model-based indicators represent fishing impacts at the whole ecosystem level.

Résumé: The effects of climate and fishing on marine ecosystems have usually been studied separately, but their interactions make ecosystem dynamics difficult to understand and predict. Of particular interest to management, the potential synergism or antagonism between fishing pressure and climate forcing is analysed in this paper, using an end-to-end ecosystem model of the southern Benguela ecosystem, built from coupling hydrodynamic, biogeochemical and multispecies fish models (ROMS-N(2)P(2)Z(2)D(2)-OSMOSE). Scenarios of different intensities of upwelling-favourable wind stress combined with scenarios of fishing top-predator fish were tested. Analyses of isolated drivers show that the bottom-up effect of the climate forcing propagates up the food chain whereas the top-down effect of fishing cascades down to zooplankton in unfavourable environmental conditions but dampens before it reaches phytoplankton. When considering both climate and fishing drivers together, it appears that top-down control dominates the link between top-predator fish and forage fish, whereas interactions between the lower trophic levels are dominated by bottom-up control. The forage fish functional group appears to be a central component of this ecosystem, being the meeting point of two opposite trophic controls. The set of combined scenarios shows that fishing pressure and upwelling-favourable wind stress have mostly dampened effects on fish populations, compared to predictions from the separate effects of the stressors. Dampened effects result in biomass accumulation at the top predator fish level but a depletion of biomass at the forage fish level. This should draw our attention to the evolution of this functional group, which appears as both structurally important in the trophic functioning of the ecosystem, and very sensitive to climate and fishing pressures. In particular, diagnoses considering fishing pressure only might be more optimistic than those that consider combined effects of fishing and environmental variability.

Résumé: We show that the EcoTroph model based on trophic spectra is an efficient tool to build ecosystem diagnoses of the impact of fishing. Using the Southern Benguela case study as a pretext, we present the first thorough application of the model to a real ecosystem. We thus review the structure and functioning of EcoTroph and we introduce the user to the steps that should be followed, showing the various possibilities of the model while underlining the most critical points of the modelling process. We show that EcoTroph provides an overview of the current exploitation level and target factors at the ecosystem scale, using two distinct trophic spectra to quantify the fishing targets and the fishing impact per trophic level. Then, we simulate changes in the fishing mortality, facilitating differential responses of two groups of species within the Southern Benguela ecosystem to be distinguished. More generally, we highlight various trends in a number of indicators of the ecosystem's state when increasing fishing mortality and we show that this ecosystem is moderately exploited, although predatory species are at their MSY. Finally, trophic spectra of the fishing effort multipliers EMSY and E(0.1) are proposed as tools for monitoring the ecosystem effects of fishing.

Résumé: Marine ecosystems have been exploited for a long time, growing increasingly vulnerable to collapse and irreversible change. How do we know when an ecosystem may be in danger? A measure of the status of individual stocks is only a partial gauge of its status, and does not include changes at the broader ecosystem level, to non-commercial species or to its structure or functioning. Six ecosystem indicators measuring trends over time were collated for 19 ecosystems, corresponding to four ecological attributes: resource potential, ecosystem structure and functioning, conservation of functional biodiversity, and ecosystem stability and resistance to perturbations. We explored the use of a decision-tree approach, a definition of initial ecosystem state (impacted or non-impacted), and the trends in the ecosystem indicators to classify the ecosystems into improving, stationary, and deteriorating. Ecosystem experts classified all ecosystems as impacted at the time of their initial state. Of these, 15 were diagnosed as “ugly”, because they had deteriorated from an already impacted state. Several also exhibited specific combinations of trends indicating “fishing down the foodweb”, reduction in size structure, reduction in diversity and stability, and changed productivity. The classification provides an initial evaluation for scientists, resource managers, stakeholders, and the general public of the concerning status of ecosystems globally.

Résumé: Background is provided to the selection of ecological indicators by the IndiSeas Working Group, and the methodology adopted for analysis and comparison of indicators across exploited marine ecosystems is documented. The selected indicators are presented, how they are calculated is explained, and the philosophy behind the comparative approach is given. The combination of selected indicators is intended to reflect different dynamics, tracking processes that display differential responses to fishing, and is meant to provide a complementary means of assessing marine ecosystem trends and states. IndiSeas relied on inputs and insights provided by the local experts from participating ecosystems, helping to understand state and trend indicators and to disentangle the effect of other potential ecosystem drivers, such as climate variability. This project showed that the use of simple and available indicators under an ecosystem approach can achieve a real, wide-reaching evaluation of marine ecosystem status caused by fishing. This is important because the socio-economics of areas where fishing activities develop differs significantly around the globe, and in many countries, insufficient data are available for complex and exhaustive analyses.

Résumé: The usefulness of indicators in detecting ecosystem change depends on three main criteria: the availability of data to estimate the indicator (measurability), the ability to detect change in an ecosystem (sensitivity), and the ability to link the said change in an indicator as a response to a known intervention or pressure (specificity). Here, we specifically examine the third aspect of indicator change, with an emphasis on multiple methods to explore the “relativity” of major ecosystem drivers. We use a suite of multivariate methods to explore the relationships between a pre-established set of fisheries-orientated ecosystem status indicators and the key drivers for those ecosystems (particularly emphasizing proxy indicators for fishing and the environment). The results show the relative importance among fishing and environmental factors, which differed notably across the major types of ecosystems. Yet, they also demonstrated common patterns in which most ecosystems, and indicators of ecosystem dynamics are largely driven by fisheries (landings) or human (human development index) factors, and secondarily by environmental drivers (e.g. AMO, PDO, SST). How one might utilize this empirical evidence in future efforts for ecosystem approaches to fisheries is discussed, highlighting the need to manage fisheries in the context of environmental and other human (e.g. economic) drivers.

Résumé: Shannon, L. J., Coll, M., Yemane, D., Jouffre, D., Neira, S., Bertrand, A., Diaz, E., and Shin, Y-J. 2010. Comparing data-based indicators across upwelling and comparable systems for communicating ecosystem states and trends. – ICES Journal of Marine Science, 67: 807-832.

Résumé: Within the IndiSeas WG, the evaluation of exploited marine ecosystems has several steps, from simple binary categorization of ecosystems to a more-complex attempt to rank them and to evaluate their status using decision-tree analyses. With the intention of communicating scientific knowledge to the public and stakeholders, focus is on evaluating and comparing the status of exploited marine ecosystems using a set of six ecological indicators and a simple and transparent graphic representation of ecosystem state (pie charts). A question that arose was whether it was acceptable to compare different types of marine ecosystems using a generic set of indicators. To this end, an attempt is made to provide reference levels to which ecosystems can be objectively compared. Unacceptable thresholds for each indicator are determined based on ecological expertise derived from a questionnaire distributed to a group of scientific experts. Analysis of the questionnaires revealed no significant difference in the thresholds provided for different ecosystem types, suggesting that it was reasonable to compare states directly across different types of ecosystem using the set of indicators selected.

Résumé: Existing models of marine ecosystems address specific issues related to the bottom-up forcing of production or to the top-down effects of fishing on a limited range of the trophic spectrum. Very few existing models explicitly incorporate the dynamics from one end of the ecosystem to the other and thus allowing the exploration of interplay between exploitation and climate effects. The shift to an ecosystem approach to fisheries and concerns about the ecological effects of climate change require the assemblage of knowledge assembled from the respective marine disciplines with the view to build end-to-end models of marine ecosystems. Here, with a focus on plankton and fish models, we present some issues and recommendations for the integration of models between trophic levels (vertical integration) and within functional groups (horizontal integration within trophic levels). At present, vertical coupling of plankton and fish models is mainly realized through predation processes, generally represented as a functional response. In the absence of empirical evidence and quantification, the choice of the functional response term is often made by default, and is reduced to a parameterization problem. A strategy is proposed to overcome this arbitrary choice. In addition to the vertical coupling of trophic models, the structure of end-to-end models incorporates biodiversity via horizontal integration of trophic levels. For guiding the selection of key components to be included in end-to-end models, the idea that marine food webs are structured as alternative trophic pathways is highlighted and related to observed dynamics. We suggest that an important early step in model development is the identification of major trophic pathways and bottlenecks in an ecosystem using a historical perspective.

Résumé: Ecosystem models provide a platform allowing exploration into the possible responses of marine food webs to fishing pressure and various potential management decisions. In this study we investigate the particular effects of overfishing on the structure and function of the southern Benguela food web, using two models with different underlying assumptions: the spatialized, size-based individual-based model, OSMOSE, and the trophic mass-balance model, Ecopath with Ecosim (EwE). Starting from the same reference state of the southern Benguela upwelling ecosystem during the 1990s, we compare the response of the food web to scenarios of overfishing using these two modelling approaches. A scenario of increased fishing mortality is applied to two distinct functional groups: i) two species of Cape hake, representing important target predatory fish, and ii) the forage species anchovy, sardine and redeye. In these simulations, fishing mortality on the selected functional groups is doubled for 10 years, followed by 10 years at the initial fishing mortality. We compare the food web states before the increase of fishing mortality, after 10 years of overfishing and after a further 10 years during which fishing was returned to initial levels. In order to compare the simulated food web structures with the reference state, and between the two modelling approaches, we use a set of trophic indicators: the mean trophic level of the community and in catches, the trophic pyramid (biomass per discrete trophic level), and the predatory/forage fish biomass ratio. OSMOSE and EwE present globally similar results for the trophic functioning of the ecosystem under fishing pressure: the biomass of targeted species decreases whereas that of their potential competitors increases. The reaction of distant species is more diverse, depending on the feeding links between the compartments. The mean trophic level of the community does not vary enough to be used for assessing ecosystem impacts of fishing, and the mean trophic level in the catch displays a surprising increase due to the short period of overfishing. The trophic pyramids behave in an unexpected way compared to trophic control theory. because at least two food chains with different dynamics are intertwined within the food web. We emphasize the importance of biomass information at the species level for interpreting dynamics in aggregated indicators, and we highlight the importance of competitive groups when looking at ecosystem functioning under fishing disturbance. Finally, we discuss the results within the scope of differences between models, in terms of the way they are formulated, spatial dimensions, predation formulations and the representation of fish life cycles.

Résumé: The main principle of the economic approach to a trophic system we propose here lies in assuming that there is a transfer of food along a path between a prey and a predator if, for the predator, the benefits are greater than costs of predation on this path. Conversely, if the costs exceed the benefits, there are no flows. This trade-off, considered all along the food chains of an ecosystem, together with ecological processes (assimilation, somatic maintenance) results in a model coupling mass balance equations (biological constraints) and complementarity principles (Walras’ law). Here is the core of the Network Economics Approach to Trophic Systems (NEATS). We illustrate with simple examples of ecosystems how these principles result in algebraic equations which can be analyzed mathematically and solved numerically.We show, in amore sophisticated example of an input/output trophic network, that they result in “affine variational inequalities”, whose solutions can be estimated. We make explicit how the approach can be applied to address ecological questions, concerning differences of productivity, causes of biological diversity, or the nature of controls in marine ecosystems.
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Résumé: Marine Protected Areas (MPAs) have been suggested as a tool that can achieve some of the goals of an Ecosystem Approach to Fisheries (EAF), e.g. prevention of overexploitation, biodiversity conservation, recovery of overexploited population, but the consequences of their establishment on the dynamics of protected components are often unclear. Spatial and multispecies models can be used to investigate the effects of their introduction. An individual-based, spatially explicit, size-structured, multispecies model (known as OSMOSE) is used to investigate the likely consequences of the introduction of three MPAs off the coast of South Africa, individually or in combination. The simultaneous introduction of the MPAs affected varying proportions of the distribution of the modelled species (5-17%) and 12% of the distribution of the whole community. In general, the introduction of the MPAs in the different scenarios resulted in a relative increase in the biomass of large predatory fish and a decrease in the biomass of small pelagic fish. The simulation demonstrates that consideration of trophic interactions is necessary when introducing MPAs, with indirect effects that may be detrimental to some (mainly smaller prey) species.