Sediment suitability of Frisian Front search areas for European flat oyster (Ostrea edulis) restoration

Summary

In the North Sea agreement an area 100 km2 at the Frisian Front will be designated for European flatoyster (Ostrea edulis) restoration. This report presents results from a field survey in two search areas, of 100 km2 each, concerning the suitability for restoration of European flat oysters. Sediment was sampled for grain size determination. High silt or very coarse sediment is considered unsuitable. In addition, pictures of the sea bed were taken to determine the presence of shells as potential settlement substrate for larvae. Based on the results of the field survey and subsequent laboratory analysis, both search areas at the Frisian Front are considered suitable for European flat oyster restoration, although the presence of shells was low. Within each location there were no clear gradients observed in presence of shells, which is considered suitable for the development of European flat oyster reefs, or low silt content, which is also considered suitable for the reef development. On average, the northern half of both locations seems to be more suitable than the southern half. Large-scale active restoration of European flat oyster beds is not without risks and presently unknown factors may influence the process. To enhance the success rate and spread the risk it is therefore advised to designate the northern half of both locations to reach the agreed total of 100 km2.