Modelling trophic flows in ecosystems to assess the efficiency of Marine Protected Area (MPA): a case study on the coast of Senegal
Résumé
Marine protected areas (MPAs) are now viewed as an efficient tool to fight against the global deterioration of habitats and fish populations. However their efficiency and effects on the whole trophic network are little known. Based on the Bolong de Bamboung (Sénégal) case study, the objective of this study is to assess the impact of a MPA on the entire trophic network using trophodynamic models. This estuarine small MPA is scientifically surveyed since 2003, one year before the closure of the fishery. Using the resulting dataset, we calculated time series of abundance indices and constructed two Ecopath mass-balanced models for the year 2003 (fished) and the 2006-2008 period (unfished). Using EcoTroph, an ecosystem trophic-level based model, we compared the two periods and simulated a closure of the fishery starting from 2003 to assess the effect of the MPA. We observed a rather constant total biomass, but a biomass increase by a 2.5 factor in predators and a decrease by a 1.7 factor in their preys. Simulations showed that the increase in predators was too important to only being a direct consequence of the removal of the fishing mortality and a local production. This highlighted the role of the MPA as a refuge or a foraging arena for some predator species. What's more, the decrease of the preys, corresponding to pelagic species, was very important and couldn't be explained only by an increase of the predation plus a release of the fishing pressure. This indicated other possible effects as environmental and behavioural ones, in addition of a direct MPA effect. Sensitivity testing and a comparison with another nearby similar but fished area, the Bolong de Sangako, validated the same global conclusion. We concluded that the fishing closure had direct effects but also indirect ones likely due to fish behaviour and environment, and that trophodynamic models are useful tools to analyse MPA effects on the whole trophic network.