European flat oysters on offshore wind farms: additional locations: opportunities for the development of European flat oyster (Ostrea edulis) populations on planned wind farms and additional locations in the Dutch section of the North Sea

To determine the relative suitability for development of self-sustaining European flat oyster (Ostrea edulis) beds, potential areas for offshore wind farms in the Dutch Exclusive Economic Zone (EEZ) were analysed following Smaal et al (2017). The studied locations are the new wind farm zones “Hollandse Kust (“noordwest”, “west” and “zuidwest”)”, “IJmuiden Ver” and the remaining lot of the North of the Frysian Islands Wind Farm Zone, but also 13 additional potential locations on the EEZ (i.e. CP4-CP6 and CP9-17) and, in addition, the nature area location Borkum Reef Ground (Borkumse Stenen). This research is performed within EZ-program Beleidsondersteunend Onderzoek (BO).
Biotic and abiotic factors of importance for flat oyster survival, growth, reproduction and recruitment, were compared for the 8 wind farm locations of Smaal et al. (2017) and the 18 new locations. For the locations on the Dutch EEZ the following habitat factors are important for flat oyster beds: shear stress, suspended sediment, larval retention, temperature, sediment composition and food availability. Presence in historic distribution area was used as a verification. Average shear stress, suspended sediment and temperature are within the range considered suitable for the development of an oyster bed at all locations. Thus, these factors do not discriminate between locations. Maximum shear stress is too high at one location and the sediment is too silty at 7 locations. However, habitat restoration efforts may make the environment more suitable, e.g. by placing shell material or 3D structures which elevate the oysters from the bottom.
Based on the analysis described in this report we recommend to select the following locations that are suitable for flat oyster restoration.
1. Best: Borssele Wind Farm Zone, Buitengaats (part of the Gemini wind park) and CP9 (within historic distribution and high larval retention);
2. Very good: Hollandse Kust (zuidwest) Wind Farm Zone (high larval retention);
3. Good: Offshore Windpark Egmond aan Zee, Prinses Amalia Windpark, Windpark Eneco Luchterduinen, Hollandse Kust (zuid) Wind Farm Zone, Hollandse Kust (noord) Wind Farm Zone (medium larval retention);
4. Suitable: the remaining lot of the North of the Frysian Islands Wind Farm Zone, Zee-energie (part of the Gemini wind park), Borkum Reef Ground (Borkumse Stenen) (within historic distribution, medium or high larval retention, but locally too high maximum shear stress or too silty);
5. Suitable with introduction of substrate: CP5, CP6 (medium or high larval retention, but too silty without introduction of substrate).