The flat oyster Ostrea edulis L., once common in the North Sea, declined rapidly due to intensive fisheries in the late 19th century and disease outbreaks at the beginning of the 20th century and is now listed as ‘threatened’ or ‘declining,’ with restoration of oyster beds now included in European directives and national plans. For oyster restoration, availability of suitable substrate is required to ensure successful settlement of oyster larvae. Off-shore windfarms are good candidates for restoration as bottom disturbance is not allowed and hard substrate is present in the form of so-called scour protection. This can provide settlement substrate for oyster larvae. In addition to the rock material that currently makes up the scour protection, studies focus on finding alternative and moldable materials that stimulate settlement. The aim of this study was to identify flat oyster larvae settlement preferences for different substrate materials. Oyster settlement on conventional scour protection rock (granite and eclogite), and currently used sandstone and concrete were compared to new types of scour protection rock (marble and limestone). In addition, three new substrates were included in the tests: a coating based on fine ground oyster shells (BESE-reef paste), substrate made of sandy dredged sediment (Geowall) and a bioinspired glue that binds crushed oyster shell fragments together (SeaCrete). Flat oyster larvae were exposed to the substrates in two hatchery experiments as well as under realistic, challenging field conditions. Flat oyster larvae settled on all substrates, with the lowest spat density on eclogite, granite and Geowall and the highest spat density on the two novel substrates SeaCrete and BESE-reef paste. These results promise to enhance native European oyster bed restoration with limited environmental impact as the novel substrates have low CO₂ footprints and make use of wasted shells from the seafood industry.

The native European flat oyster (Ostrea edulis) is an ecosystem engineer providing important ecosystem services, but became nearly extinct from the North Sea due to diseases and overfishing. There's a growing interest to restore these oyster reefs for their valuable contribution in re-establishing a rich ecosystem in the North Sea. In order to reintroduce the flat oyster population, the availability of hard substrate is crucial for initial settlement and reef development. Such substrate is offered by the infrastructure in offshore wind farms, by means of quarried rock placed at the base of the wind turbine foundations and on top of cable crossings to prevent scouring of the seabed. Further anthropogenic disturbances of the seabed are largely restricted, making wind farm areas promising sites for oyster reef restoration. For successful oyster reef initiation, offering a suitable type of substrate for larvae settlement is important. Here, we assess the settlement preference of flat oysters on 9 different types of substrate, by comparing total settlement, spat densities and spat survival. Oyster larvae settlement preference based on the total number of spat per surface area of the substrate was the highest for granite, a rock type conventionally used as scour protection in offshore wind farms. The lowest settlement preference was observed for steel and the biodegradable polymer BESE. The experiments were performed in a spatting pond and in a natural bay to be able to compare spat collection under controlled and natural conditions. Settlement rates in the spatting pond were much higher than in the natural environment, though survival rates were lower. Our results provide insight in the settlement preference of the European flat oyster for different types of substrate under controlled and natural conditions. Knowing these favorable substrates and conditions for oyster larvae settlement allows for the selection of pro-active measures that contribute to flat oyster reef restoration in the North Sea.