Economic viability assessment of European flat oyster restoration on offshore windfarm infrastructure

European oysters ( Ostrea edulis ) once covered large areas of the North Sea, but have disappeared due to a combination of overexploitation and the destruction of benthic habitats including hard settlement substrate. Offshore wind parks offer an opportunity for oyster restoration as fishing is banned inside these parks and scour protection provides hard settlement substrate. However, ecological restoration of marine systems is capital-intensive. The success of restoration projects is mainly determined by the choice of methods and techniques and consequently costs. Costs and cost-effectiveness information are therefore key in decision making processes concerning the selection of restoration efforts and techniques. So far, economic viability of marine ecosystem restoration have mainly focused on near-shore shallow habitats. The aim of this study was to provide insight into the most cost-effective deployment options to create a European flat oyster reef in an offshore wind farm in the North Sea. Within the current policy and legislation framework, several deployment scenarios were identified based on best practices, expert knowledge, and preliminary results of several pilots. The 9 scenarios included ‘adults placed loose on the seafloor’, ‘adults glued on granite’, ‘spat settled on shells’, ‘spat settled on granite’ and a combined ‘adult and spat’ scenario. Cost-effectiveness of the different scenarios was determined by modelling the expected reef biomass post-deployment both with and without the option to add additional settlement substrate post-deployment. The main conclusions from this exercise were that: 1. based on investment value, the scenarios ‘adult loose on the seafloor’, ‘adults in cages’ and ‘spat on shells’ had the highest revenues per Euro invested; 2. adding substrate in the years post-deployment increased cost-effectiveness in the model for all scenarios, and 3. the time post-deployment to reach a self-sustaining adult oyster population was, with 8–10 yr, shortest for the scenarios 'spat settled on shells' and the combined scenario of 'adults placed loose on the seafloor' and 'spat settled on shells'.