Which concrete substrate suits you? Ostrea edulis larval preferences and implications for shellfish restoration in Europe

The European flat oyster, Ostrea edulis, is an important ecosystem engineer that has been progressively disappearing from European coasts over the last century mainly due to overexploitation, habitat degradation and disease. It is now the subject of many conservation and restoration programs throughout Europe, including the Flat Oyster REcoVERy (FOREVER) project in France. Protecting and managing the remaining populations has become a nature conservation priority because this species is able to build biogenic reefs, very specific habitats that provide many ecosystem functions and services. The availability of suitable hard substrates for larval fixation is a critical factor during this reef-building process. Although natural substrates are in short supply, production and deployment is an easy step to help oyster reef restoration. The present study was carried out to improve artificial reef design and the composition of the concrete used to build them, focusing on the impact of concrete formulation and surface texture on larval settlement in the field. Nine bio-sourced concrete formulations and ten surface textures were evaluated. The number of settled larvae counted on each concrete substrate reflected their preferences, and results demonstrated that microscale surface texture has a greater impact on recruitment than concrete formulation, with larvae preferring to settle in depressions on a rough rock-like texture and avoiding flat, horizontal and exposed areas. Physical and mechanical properties of the different formulations were also evaluated. Although they had almost no impact on recruitment, these results could be helpful for artificial reef-building (in terms of 3D design, durability, manufacture and deployment) and guide choices of materials that should be used and the proportion and granulometry of incorporated shells to obtain the best surface texture. •Ostrea edulis larval preferences for different substrates were evaluated in situ.•Various bio-sourced concrete mix formulations and surface textures were tested.•Incorporation of seashell by-products influence mechanical properties of concrete.•Substrate texture has a greater impact on larval settlement than its formulation.•Substrate micro-topography has direct implications for flat oyster restoration.