Abstract
European flat oysters (Ostrea edulis), important ecosystem engineers with several important ecosystem services, have near to disappeared from the Dutch North Sea. In order to reintroduce the flat oyster population in the North Sea, the availability of hard substrate for initial settlement is critical. Such substrate is offered in offshore wind farms where further anthropogenic disturbances of the seabed are restricted, making them a promising site for restoration efforts. Nature inclusive designs of scour protection around the base of the monopiles in wind farms could greatly improve the ecosystem contribution of offshore wind farms. However, the best type of substrate to use as scour protection to aid oyster settlement is unknown. In the project EcoScour(Protection), settlement success of oysters on different types of substrates is compared in order to find the best substrate for application in an eco-friendly scour protection in offshore wind farm Borssele V. The substrate types tested are: fresh mussel shell, weathered mussel shell, granite, sandstone, silex, marble, concrete, ECOncrete, roof tile, steel and BESE-elements. Settlement success is compared using three different measures: 1) total numbers of spat settled - in order to compare total spat collected per type of substrate provided in identical baskets, 2) settlement per kg of substrate - to find optimal substrates with regards to transport, and 3) settlement per cm2 of substrate - to compare success between substrates independent of the available space to settle on. The substrates were deployed at three different locations; the saltwater lake Grevelingen in the Netherlands, an oyster spatting pond in New Quay, Ireland and a natural bay in Tralee, Ireland. In the Grevelingen settlement of oyster spat per basket was most successful on weathered mussel shells, while granite was most successful at both locations in Ireland. Per kg fresh mussel shells were the most successful substrate at all locations. In terms of spat collection per cm2, granite (New Quay and Tralee) and marble (lake Grevelingen) were also successful and more so than mussel shells. The final choice of substrate for application in wind farms can depend on several factors. (1) When considering number of spat per mass of substrate, mussel shells are the substrate of choice. (2) When taking into account the cost-effectiveness for application in eco-friendly scour protection, working with construction materials such as granite or (E)concrete or by-products such as silex or marble (both from quarrying) are good options. (3) In view of longevity of the ecosystem and persistence even after decommissioning of the wind farm, substrates that are originally found in the North Sea could be considered. For that purpose, mussel shell or (red) granite gravel would be the best choice of oyster spat collection substrates to apply in eco-friendly scour protection.