Introduction
Growing demand using European seas for renewable energy, food production, tourism and transportation has resulted in increasing competition for marine space with the push to move into offshore waters. The Belgian pilot within the European Union’s Horizon 2020 project UNITED1 (Multi-Use offshore platforms demoNstrators for boostIng cost-effecTive and Eco-friendly proDuction in sustainable marine activities) seeks to combine the cultivation of seaweed (Saccharina latissima) and European flat oyster (Ostrea edulis), and the restoration of flat oyster reefs within an offshore windfarm operated by Parkwind. To do so, best cultivation and restoration practices to withstand the harsh environmental conditions were identified in the nearshore test site at Westdiep, a NATURA 2000 area five kilometres off the coast of Nieuwpoort. Only the best suited materials and methods will be applied in the offshore site in the Belwind offshore wind farm, 46 km off the coast of Zeebrugge.
Materials and methods
The installation of the four offshore oyster restoration structures was conducted early summer 2021. Four restoration structures filled with nature-inclusive scour materials were placed on the scour protection of two monopiles. At each monopile, two structures were installed. At the first monopile, adult European flat oysters (Ostrea edulis) were added. At the monopile downstream of the first one (with respect to the main current direction),
no oysters were added. The goal of this is to verify whether settlement on the scour protection materials and reef restoration can be enhanced by adding adult oysters.
For the flat oyster cultivation, several techniques were tested, such as oysters cemented or glued to ropes and sticks, and OSTRIGA lantern baskets connected to a rope.
To identify best cultivation practices for Saccharina latissima (known as sugar kelp) in the highly exposed environment of the Belgian part of the North Sea (BPNS), different substrates, seeding techniques (comparing direct seeding versus the importance of a nursery period), and strains were investigated. Two types of seaweed cultivation nets (AtSeaNova) were partly pre-seeded with two different strains of S. latissima gametophytes and kept in a nursery setting for four weeks. The remaining part of the nets was seeded with the direct seeding method a day before being installed at the nearshore site (Westdiep) in November 2020. Biomass, density, maximum length and width of S. latissima were assessed upon the harvest in May 2021.
Results
The offshore restoration of O. edulis reefs gave very promising results. Successful settlement of juvenile flat oysters was discovered on both the scour protection and the added shell materials.
Concerning oyster cultivation nearshore, the oysters cemented to ropes initially seemed to be a practical cultivation method. However, the ropes were overgrown with fouling organisms, mostly mussels once the mussel spat season had begun and water temperatures had risen. The OSTRIGA baskets on the other hand proved to be successful, although pinpointing of the amount of stacks, maze width, and depth was necessary. The other systems seemed to be unpractical (too heavy, basket loss, too much fouling) and need further tweaking before commercial upscaling and/or implementation in a wind farm can be considered.
The first seaweed cultivation trials resulted in a promising harvest of S. latissima in May 2021 (Figure 2). While seaweed biomass was of good quality with barely any visible epiphytes, strong biofouling especially on substrates seeded via direct seeding was observed, resulting in low biomass (0.1 kg m-1). Best results were achieved with nets incubated in the nursery before deployment (1.1 kg m-1).
Discussion and conclusion
Overall, the implementation of offshore multi-use activities is still in its infancy. Test pilots such as the Belgian pilot are a prerequisite before offshore multi-use can take place. While nearshore testing of systems and techniques are needed before moving offshore, the real test will come with the installation of offshore seaweed and oyster aquaculture inside an offshore wind farm. The first results of the offshore oyster reef structures in the Belgian pilot show at least that this environment is suitable for the survival, growth, reproduction and settlement of European flat oysters, which looks promising for successfully implementing oyster restoration inside offshore wind farms in the BPNS. The first seaweed cultivation trials made clear that proven cultivation protocols from sheltered locations need adaptation for the successful implementation in the highly exposed offshore environment. Future planned activities on the installation of the aquaculture structures offshore will prove possible further success.
References
UNITED1: Multi-Use offshore platforms demoNstrators for boostIng cost-effecTive and Eco-friendly proDuction in sustainable marine activities. Retrieved April 26, 2022, from Offshore wind and flat oyster aquaculture & restoration in Belgium (h2020united.eu)