The feasibility of native oyster restoration in the Belgian part of the North Sea
The European flat oyster Ostrea edulis has been a species of great ecological and commercial importance for many centuries all over Europe. Historical records indicate a widespread abundance of natural O. edulis beds in the southern North Sea, extending for kilometres in length even far offshore. Years of over-exploitation in combination with disease outbreaks, harsh winters, habitat degradation and the species’ own sensitivity to altering environmental conditions has led to a serious population decline, to the point of near extinction. It is now considered a threatened and/or declining species and threatened and/or declining habitat in all OSPAR regions. Oyster beds are now protected as ‘biogenic reefs’ under the EU Habitats Directive.
Figure 1. Alive Ostrea edulis individual (attached to a stone) found during a monitoring campaign at the Hinder Banks in July of 2018. Photo credit: RBINS.
In response to the state of the species and latest legislative measures, several Ostrea edulis restoration and management projects have been initiated across Europe. Accordingly, a study was carried out in 2018 to investigate the feasibility of native oyster restoration in the Belgian part of the North Sea. The main objectives were:
- to locate areas with suitable environmental conditions
- to investigate connectivity potential
- to identify potential threats to restoration
- to develop oyster reef restoration scenarios
- to propose focal sites for oyster reef restoration
The study demonstrated that suitable habitats for O. edulis still exist in Belgian waters and that there is enough larval flow from nearby populations to allow for genetic exchange between natural populations and restored habitats in the future. Recent sightings of live or recently alive O. edulis individuals (Kerckhof, 2018) also indicate that native oysters still settle and may even reproduce in Belgian waters.
In deciding on the most promising sites for ecosystem restoration sites habitat suitability will be important. However, it will be necessary also to choose areas where human impacts are low and manageable. Most importantly, these areas should be completely protected from activities which might disturb the seabed, such as bottom trawling but also dredging, dumping and aggregate extraction.
Restoration scenarios have focused on the means of oyster introduction (passive colonisation versus active re-introduction) and possibilities for habitat modification (introduction of gravels, shell hash or artificial structures). These scenarios were scored using the following criteria: the time required to ensure successful restoration, the impact on the existing biodiversity, the facilitation of new biodiversity, and the restoration and maintenance costs.
Figure 2. Close-up of the Ostrea edulis individual from the photo above. The oyster is attached to a stone that is hosting several other organisms such as the tube-building polychaete Spirobranchus triqueter, the soft coral Alcyonium digitatum, small Hydrozoan and Bryozoan colonies etc. Photocredit: RBINS.
Considering all criteria, three potential areas were identified for restoration purposes: 1) the gravel beds area of the Hinder Banks, 2) the existing wind farm zone near the Dutch-Belgian border and 3) the newly proposed wind farm zone near the Hinder Banks.The gravel beds seemed to be the most promising in terms of habitat suitability, provided that impacts from bottom fisheries could be excluded. Despite having reached an agreement with the concerned member states, the European Parliament decided against limitations on bottom trawling in the Belgian part of the North Sea, under pressure from both environmental and fisheries organisations. At present, no further action towards an O. edulis restoration can be taken, but the newly proposed offshore wind farm area, with the advantage of creating de factofisheries exclusion zones in Belgium, may offer new possibilities for oyster bed restoration in the coming years.
Further information
Steven Degraer (sdegraer@naturalsciences.be) and Ilse De Mesel (idemesel@naturalsciences.be), Royal Belgian Institute of Natural Sciences
Further reading
De Mesel I., D. Kapasakali, F. Kerckhof, L. Vigin, G. Lacroix, L. Barbut and S. Degraer (2018). Ostrea edulisrestoration in the Belgian part of the North Sea: Feasibility study. Royal Belgian Institute of Natural Sciences, Operational Directorate Natural Environment, Marine Ecology and Management. pp. 89.
Kerckhof F. (2018). Recente strandvondsten van inheemse platte oesters Ostrea edulisLinnaeus, 1758 op de stranden van de westkust en het nut van lege schelpen van de Amerikaanse zwaardschede Ensis directus. De Strandvlo38(4): 112-118. (in Dutch)
Kerckhof F., Coolen J.W.P., Rumes B. and S. Degraer (2018). Recent findings of wild European flat oysters Ostrea edulis(Linnaeus, 1758) in Belgian and Dutch offshore waters: New perspectives for offshore oyster reef restoration in the southern North Sea. Belgian Journal of zoology148(1): 13–24.
https://doi.org/10.26496/bjz.2018.16
https://www.naturalsciences.be/en/science/home
