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Ba, K., Thiaw, M., Fall, M., Thiam, N., Meissa, B., Jouffre, D., et al. (2018). Long-term fishing impact on the Senegalese coastal demersal resources: diagnosing from stock assessment models. Aquat. Living Resour., 31, 8.
Résumé: For the first time in Senegal, assessments based on both stochastic and deterministic production models were used to draw a global diagnosis of the fishing impact on coastal demersal stocks. Based one national fisheries databases and scientific trawl surveys data: (i) trends in landings since 1971 were examined, (ii) abundance indices of 10 stocks were estimated using linear models fitted to surveys data and commercial catch per unit efforts, and (iii) stock assessments were carried out using pseudo-equilibrium Fox and Pella-Tomlinson models and a Biomass dynamic production model fitted in a Bayesian framework to abundance indices. Most stocks have seen their abundance sharply declining over time. All stocks combined, results of stock assessments suggest a 63% reduction compared to virgin state. Three fifth of demersal stocks are overexploited and excess in fishing effort was estimated until 75% for the worst case. We conclude by suggesting that the fishing of such species must be regulated and an ecosystem approach to fisheries management should be implemented in order to monitor the whole ecosystem.
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de Fouw, J., Govers, L. L., van de Koppel, J., van Belzen, J., Dorigo, W., Cheikh, M. A. S., et al. (2016). Drought, Mutualism Breakdown, and Landscape-Scale Degradation of Seagrass Beds. Curr. Biol., 26(8), 1051–1056.
Résumé: In many marine ecosystems, biodiversity critically depends on foundation species such as corals and seagrasses that engage in mutualistic interactions [1-3]. Concerns grow that environmental disruption of marine mutualisms exacerbates ecosystem degradation, with breakdown of the obligate coral mutualism (“coral bleaching”) being an iconic example [2, 4, 5]. However, as these mutualisms are mostly facultative rather than obligate, it remains unclear whether mutualism breakdown is a common risk in marine ecosystems, and thus a potential accelerator of ecosystem degradation. Here, we provide evidence that. drought triggered landscape-scale seagrass degradation and show the consequent failure of a facultative mutualistic feedback between seagrass and sulfide-consuming lucinid bivalves that in turn appeared to exacerbate the observed collapse. Local climate and remote sensing analyses revealed seagrass collapse after a summer with intense low-tide drought stress. Potential analysis a novel approach to detect feedback-mediated state shifts-revealed two attractors (healthy and degraded states) during the collapse, suggesting that the drought disrupted internal feedbacks to cause abrupt, patch-wise degradation. Field measurements comparing degraded patches that were healthy before the collapse with patches that remained healthy demonstrated that bivalves declined dramatically in degrading patches with associated high sediment sulfide concentrations, confirming the breakdown of the mutualistic seagrass-lucinid feedback. Our findings indicate that drought triggered mutualism breakdown, resulting in toxic sulfide concentrations that aggravated seagrass degradation. We conclude that external disturbances can cause sudden breakdown of facultative marine mutualistic feedbacks. As this may amplify ecosystem degradation, we suggest including mutualisms in marine conservation and restoration approaches.
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van der Geest, M., van Gils, J. A., van der Meer, J., Olff, H., & Piersma, T. (2011). Suitability of calcein as an in situ growth marker in burrowing bivalves. Journal of Experimental Marine Biology and Ecology, 399, 1–7.
Résumé: The fluorochrome calcein has been used in numerous growth studies of molluscs to internally mark calcified structures. Because of interspecific variations in marking success and possible effects on growth performance, methodological assessments of the suitability of calcein as a growth marker, especially in field contexts, remain necessary. Here we report on the effects of different calcein concentrations (100, 200, 400 and 800 mg l(-1) on fluorescent mark deposition, growth rate, density, body condition and size-frequency distribution of an intertidal infaunal bivalve species, Loripes lacteus (Linnaeus, 1758), using an outdoor immersion technique. To avoid stress caused by handling and transportation, in situ enclosures were placed at seagrass-covered patches during low tide, to which calcein solutions were added. After a 1.3- to 2.6-h period of exposure to calcein, the enclosures were removed. Sites were sampled three months later. All calcein concentrations produced live L. lacteus with a clear fluorescent shell mark, but the percentage successfully marked tended to increase with higher calcein concentrations. Furthermore, marking success and growth rate decreased significantly with shell size (i.e., age). Calcein concentration did not measurably affect shell growth rate, body condition and size-frequency distribution, but the numerical densities were lower for sites treated with calcein concentrations >= 400 mg l(-1). Our results suggest that in situ calcein-marking of burrowing lucinid bivalves with low concentrations (100-200 mg l(-1)) is a non-invasive and rapid method to determine growth rate, provided that the lucinids are not too old. (c) 2011 Elsevier B.V. All rights reserved.
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van Gils, J. A., van der Geest, M., Jansen, E. J., Govers, L. L., de Fouw, J., & Piersma, T. (2012). Trophic cascade induced by molluscivore predator alters pore-water biogeochemistry via competitive release of prey. Ecology, 93, 1143–1152.
Résumé: Effects of predation may cascade down the food web. By alleviating interspecific competition among prey, predators may promote biodiversity, but the precise mechanisms of how predators alter competition have remained elusive. Here we report on a predator-exclosure experiment carried out in a tropical intertidal ecosystem, providing evidence for a three-level trophic cascade induced by predation by molluscivore Red Knots (Calidris canutus) that affects pore water biogeochemistry. In the exclosures the knots' favorite prey (Dosinia isocardia) became dominant and reduced the individual growth rate in an alternative prey (Loripes lucinalis). Dosinia, a suspension feeder, consumes suspended particulate organic matter (POM), whereas Loripes is a facultative mixotroph, partly living on metabolites produced by sulfur-oxidizing chemoautotrophic bacteria, but also consuming suspended POM. Reduced sulfide concentrations in the exclosures suggest that, without predation on Dosinia, stronger competition for suspended POM forces Loripes to rely on energy produced by endosymbiotic bacteria, thus leading to an enhanced uptake of sulfide from the surrounding pore water. As sulfide is toxic to most organisms, this competition-induced diet shift by Loripes may detoxify the environment, which in turn may facilitate other species. The inference that predators affect the toxicity of their environment via a multi-level trophic cascade is novel, but we believe it may be a general phenomenon in detritus-based ecosystems.
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