Using trophic models to assess the impact of fishing in the Bay of Biscay and the Celtic Sea

Using the Bay of Biscay and Celtic Sea area as a case study, we showed how stock-assessments and trophic models can be useful and complementary tools to quantify the fishing impacts on the whole food web and to draw related diagnoses at the scale of marine ecosystems. First, an integrated synthesis of the status and trends in fish stocks, derived from ICES assessments, was consolidated at the ecosystem level. Then, using the well-known Ecopath and Ecosim and the more recently developed EcoTroph approach, we built advice-oriented ecosystem models structured around the stocks assessed by ICES. We especially analysed trends over the last three decades and investigated the potential ecosystem effects of the recent decrease observed in the overall fishing pressure. The Celtic/Biscay ecosystem appeared heavily fished during the 1980-2015 period. Some stocks would have started to recover recently, but changes in species composition seem to lead to more rapid and less efficient transfers within the food web. This could explain why the biomass of intermediate and high trophic levels increased at lower rates than anticipated from the decrease in the fishing pressure. We conclude that, in the frame of the Ecosystem approach to fisheries management, trophic models are key tools to expand stock assessment results at the scale of the whole ecosystem, and to reveal changes occurring in the global parameters of the trophic functioning of ecosystems.