Although the formal process of Brexit has started, there is high uncertainty about Brexit’s impacts on the future of demersal mixed fisheries in the North Sea. As migratory demersal stocks (e.g. cod, haddock, hake and saithe) traverse the zones of various countries, one challenging issue is to agree on how catch shares and management responsibilities of fish stocks will be allocated between the United Kingdom and the European Union after Brexit. Depending on the agreed scheme, fishers may need to adjust and change their fishing behavior in response to economic and management incentives, which in turn will lead to different fishery outcomes. This study uses a bio-economic simulation and optimization model that considers responses of fishers to management schemes including fleet economics, the impact of fishing on stock development and their spatio-temporal interplay. The model is used to explore how three different negotiation strategies in combination with species seasonal movements affect levels of catch, discards, net profit of individual fleet segments, and stock development in that fishery. Costs and benefits are assessed under a soft (unchanged access to British waters and current quota distribution key), a moderate (no access to British waters and current quota distribution key) and a hard Brexit scenario, where EU vessels are excluded from British waters and quota distribution is based on the zonal attachment principle (distribute quota based on how much of a fish stock resides in a country’s zone).