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Brosset, P., Le Bourg, B., Costalago, D., Banaru, D., Van Beveren, E., Bourdeix, J. - H., et al. (2016). Linking small pelagic dietary shifts with ecosystem changes in the Gulf of Lions. Mar. Ecol.-Prog. Ser., 554, 157–171.
Résumé: Since 2008, a severe decrease in size and body condition together with a demographic truncation has been observed in the sardine (secondarily in anchovy) population of the Gulf of Lions (NW Mediterranean Sea). In parallel, sprat biomass, which was negligible before, has increased tenfold. All of these changes have strongly affected the regional fisheries. Using trophic and isotopic data from contrasting periods of low versus high growth and condition, we investigated potential changes in diet and interspecific feeding interactions through time. Evidence of resource partitioning was found between sprat and both anchovy and sardine in 2004 and 2005. Since 2010, the isotopic niches of the 3 species have tended to overlap, suggesting higher risk of competition for food resources. Moreover, the wider trophic niche of sprat indicates higher variability in individual diets. Anchovy and sardine diet varied through time, with a high proportion of large copepods or cladocerans in periods of high growth and condition (1994 and 2007, respectively) versus a dominance of small copepods in the present (2011-2012). Furthermore, an important reduction in prey diversity was also identified in the diet of both anchovy and sardine during the most recent period. Our results support the hypothesis that changes in small pelagic fish growth, size and body condition and ultimately biomass could be due to bottom-up control characterized by changes in food availability and increasing potential trophic competition.
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Gasche, L., Gascuel, D., Shannon, L., & Shin, Y. - J. (2012). Global assessment of the fishing impacts on the Southern Benguela ecosystem using an EcoTroph modelling approach. Journal of Marine Systems, 90(1), 1–12.
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.
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Heymans, J. J., Coll, M., Link, J. S., Mackinson, S., Steenbeek, J., Walters, C., et al. (2016). Best practice in Ecopath with Ecosim food-web models for ecosystem-based management. Ecol. Model., 331, 173–184.
Résumé: Ecopath with Ecosim (EwE) models are easier to construct and use compared to most other ecosystem modelling techniques and are therefore more widely used by more scientists and managers. This, however, creates a problem with quality assurance; to address this we provide an overview of best practices for creating Ecopath, models. We describe the diagnostics that can be used to check for thermodynamic and ecological principles, and highlight principles that should be used for balancing a model. We then highlight the pitfalls when comparing Ecopath models using Ecological Network Analysis indices. For dynamic simulations in Ecosim we show the state of the art in calibrating the model by fitting it to time series using a formal fitting procedure and statistical goodness of fit. Finally, we show how Monte Carlo simulations can be used to address uncertainty in input parameters, and we discuss the use of models in a management context, specifically using the concept of 'key runs' for ecosystem-based management. This novel list of best practices for EwE models will enable ecosystem managers to evaluate the goodness of fit of the given EwE model to the ecosystem management question. (C) 2016 The Authors. Published by Elsevier B.V.
Mots-Clés: benguela ecosystem; dynamics; Ecological network analysis; Ecopath with Ecosim; Ecosystem-based management; Ecosystem modelling; exploited ecosystems; impacts; indicators; marine ecosystems; Monte Carlo; network analysis; nw mediterranean sea; shelf ecosystem; southern benguela; Time series fitting
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Kirkman, S. P., Blamey, L., Lamont, T., Field, J. G., Bianchi, G., Huggett, J. A., et al. (2016). Spatial characterisation of the Benguela ecosystem for ecosystem-based management. Afr. J. Mar. Sci., 38(1), 7–22.
Résumé: The three countries of the Benguela Current Large Marine Ecosystem (BCLME), namely Angola, Namibia and South Africa, have committed to implementing ecosystem-based management (EBM) including an ecosystem approach to fisheries (EAF) in the region, to put in practice the principles of sustainable development in ocean-related matters. There is also recognition of the need for marine spatial planning (MSP) as a process for informing EBM with regard to the allocation and siting of ocean uses so that ecosystem health is ensured and trade-offs between ecosystem services are appropriately dealt with. Marine spatial planning is both an integrated and an area-based process, and this paper produces a spatial characterisation of the BCLME for achieving a common basis for MSP in the region, focusing on the oceanography, biology and fisheries. Recognising spatial variation in physical driving forces, primary and secondary production, trophic structures and species richness, four different subsystems are characterised: (1) north of the Angola-Benguela Front, (2) from the Angola-Benguela Front to Luderitz, (3) from Luderitz to Cape Agulhas, and (4) from Cape Agulhas to Port Alfred on the south-east coast of South Africa. Research and monitoring requirements of relevance for MSP and EBM in the region are identified, focusing on understanding variability and change, including with regard to the boundary areas identified for the system. To this end, 14 cross-shelf monitoring transects are proposed (including seven that are already being monitored) to estimate fluxes of biota, energy and materials within and between the subsystems. The usefulness of models for understanding ecosystem variability and changes is recognised and the need for fine-scale resolution of both sampling and modelling for adequate MSP as input to EBM for the often-conflicting interests of conserving biodiversity, and managing fisheries, recreation, offshore oil and gas exploration and exploitation, offshore mining and shipping routes, is emphasised.
Mots-Clés: agulhas system; atlantic-ocean; biology; drivers; ecosystem approach to fisheries; large marine ecosystem; large marine ecosystem; marine spatial planning; models; monitoring; northern benguela; nursery areas; physical oceanography; regime shifts; sea use management; southern benguela; upwelling system; variability
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Sabarros, P. S., Grémillet, D., Demarcq, H., Moseley, C., Pichegru, L., Mullers, R. H. E., et al. (2014). Fine-scale recognition and use of mesoscale fronts by foraging Cape gannets in the Benguela upwelling region. Deep Sea Research Part II: Topical Studies in Oceanography, , 77–84.
Résumé: Oceanic structures such as mesoscale fronts may become hotspots of biological activity through concentration and enrichment processes. These fronts generally attract fish and may therefore be targeted by marine top-predators. In the southern Benguela upwelling system, such fronts might be used as environmental cues by foraging seabirds. In this study we analyzed high-frequency foraging tracks (GPS, 1 s sampling) of Cape gannets Morus capensis from two colonies located on the west and east coast of South Africa in relation to mesoscale fronts detected on daily high-resolution chlorophyll-a maps (MODIS, 1 km). We tested the association of (i) searching behavior and (ii) diving activity of foraging birds with mesoscale fronts. We found that Cape gannets shift from transiting to area-restricted search mode (ARS) at a distance from fronts ranging between 2 and 11 km (median is 6.7 km). This suggests that Cape gannets may be able to sense fronts (smell or vision) or other predators, and that such detection triggers an intensified investigation of their surroundings (i.e. ARS). Also we found that diving probability increases near fronts in 11 out of 20 tracks investigated (55%), suggesting that Cape gannets substantially use fronts for feeding; in the remaining cases (45%), birds may have used other cues for feeding including fishing vessels, particularly for gannets breeding on the west coast. We demonstrated in this study that oceanographic structures such as mesoscale fronts are important environmental cues used by a foraging seabird within the rich waters of an upwelling system. There is now need for further investigations on how Cape gannets actually detect these fronts.
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