Abstract
Shellfish reefs, mainly European flat oysters (Ostrea edulis), once occurred in 30% of the Dutch part of the North Sea. They have almost completely disappeared, due to overfishing, habitat destruction and diseases. Wind farms have been recognized as potential sites for the introduction of flat oysters, as they provide hard substrate and an undisturbed seabed due to the exclusion of trawling fishing boats.
In 2018 Stichting de Noordzee, Natuur en Milieu, Eneco, van Oord and ASN started a pilot on nature enhancement with flat oysters in offshore Windpark Eneco Luchterduinen (LUD).
The general objective of the LUD pilot is to elucidate how offshore nature enhancement, in particular with oyster reefs, can successfully be carried out in an offshore wind farm. The main objective is to identify key factors for success or failure in Windpark Eneco Luchterduinen. The second objective is to define what lessons learned from the pilot mean for the scalability of the used methods for the nature enhancement in offshore wind farms in the North Sea.
The results show that in the wind farm Luchterduinen survival, growth and reproduction (gonad development, larvae, swarming larvae in high concentrations) of introduced Bonamia free flat oysters from Norway was achieved in 2019 and that oysters remained Bonamia free. Furthermore, the installations attracted substantial biodiversity. These results indicate Luchterduinen wind farm can be a suitable site for flat oyster restoration and nature enhancement in general.
Considering the racks were found semi-submerged into the sea bed and the likelihood of these conditions reoccurring was deemed very high, the decision was made to interrupt and demobilise the pilot to prevent reoccurrence of this same phenomenon. In conclusion:
- Use of adult Bonamia free Norwegian oysters to serve as broodstock in offshore wind farms show promising results on a biological level with respect to reproduction and swarming larvae as a result of this 8-month pilot.
- Sand accretion was observed around and in the oyster rack in the high dynamic conditions of Luchterduinen.
- It is unclear whether the racks have sunk into the sand, or whether the sand has been accumulated into it. The design of the oyster racks should be revised for further testing at this location, or comparable sites.
- The sand accretion also indicates that the dynamic conditions in LUD need to be reviewed, especially in the context of the scalability of the pilot.
The next step in offshore oyster pilots should include redesigning equipment such as the oyster racks and monitor if the observed larvae in the water column have resulted in recruitment and survival of young oysters within the wind farm.