A stochastic bio-economic model for the viable management of the Bay of Biscay mixed demersal fisheries S. Gourguet, a,b,c,∗ , C. Macher b , L. Doyen a , O. Th´ ebaud c , M. Bertignac d , O. Guyader b a CNRS-MNHN, CERSP UMR 7204, CP 51, 55 rue Buffon, 75005, Paris, France b IFREMER, UMR AMURE, D´ epartement d’Economie Maritime, BP 70,29280 Plouzan´ e, France c CSIRO, Marine and Atmospheric Research, Ecoscience Precinct, 41 Boggo Road, Dutton Park, QLD 4102, Australia d IFREMER, D´ epartement Sciences et Technologies Halieutiques, Laboratoire de Biologie des Pˆ echeries, BP 70,29280 Plouzan´ e, France Abstract Marine fisheries resources are under extreme pressure worldwide. Marine scientists and stakeholders advocate ecosystem-based fishery management (EBFM) for an effective and sustainable management. However, the way to operationalize such EBFM remains controversial. The stochastic co-viability approach can be a rele- vant modeling framework for EBFM as it accounts for dynamic complexities, un- certainties, risks and sustainability objectives balancing ecological, economic and social dimensions together with intergenerational equity. The present paper focuses on the case of the mixed demersal fisheries operating in the Bay of Biscay and especially harvesting Nephrops (Nephrops norvegicus ), Hake (Merluccius merluc- cius ) and Sole (Solea solea ). A bio-economic multi-species and multi-fleets model is developed to examine how to preserve Spawning Stock Biomass (SSB) for every species while preserving the economic profitability for the various fleets at play. First results suggest that the viable strategies require a significant mitigation of the fishing capacities as compared to referenced year 2008 for Nephrops trawlers and gill-netters fleets that are the most contributory fleets to Nephrops and Sole fishing mortality. Key words: Fisheries, ecosystem-based management, bio-economic model, co-viability, uncertainty, Bay of Biscay ∗ Corresponding author at: MNHN, CERSP UMR 7204, CP 51, 55 rue Buffon,75005 Preprint submitted to 13th Annual BIOECON Conference May 11, 2011