Oyster reefs: a lost habitat in the Belgian part of the North Sea
Historical records show that, in the past, extensive reefs of the European flat oyster Ostrea edulis (Linnaeus, 1758) occurred on and near the gravel reefs of our North Sea. The oyster reefs were inhabited by a lot of species that lived on and among the oyster shells, such as tube-forming bristle worms and organisms like the sea sprite or dead man's thumb. All these organisms together formed a three-dimensional structure between which lived numerous other smaller and more mobile species. Thanks to their complexity, oyster reefs formed the hotspots for biodiversity in our North Sea.
Disturbance of oyster reefs
Restoration of oyster reefs
Objectives
Research and project start-up(year0-1)
Test phase(year 1-3)
Expansion phase(year 3-6)
Development phase(year 6-10)
Oyster Bank© Floor Driessen/Bureau Waardenburg
The European flat oyster used to be present in large numbers along almost all European coasts (Figure 2). Among others, large oyster reefs occurred on the Hinderbanken. Over the last 150 years, however, the number of oysters declined dramatically. Between 1868 and 1873, Belgian oyster reefs were so heavily exploited that they virtually disappeared. Moreover, the species was affected by the oyster parasite Bonamia ostreae, which was imported into Europe from California via oyster transport around 1979. In France, the Netherlands and Belgium, the remaining European flat oysters almost completely disappeared as a result of this parasite.
Distribution of the European oyster, map from Olsen's "Piscatorial Atlas" (1883)
The species is therefore not doing well outside Belgian waters either, which means that the natural populations of the European flat oyster are virtually extinct throughout the Southern Bight of the North Sea. Consequently, the flat oyster is on several lists of declining and endangered species and national actions for protection and recovery are being encouraged by various international initiatives.
Disturbance of oyster reefs
Recent research has shown that some larvae of the European flat oyster are still entering our North Sea and that environmental conditions are still suitable for the settlement and growth of this species. However, the absence of undisturbed, natural hard substrate such as gravel reefs and shell gravel appears to be a limiting factor for the effective recovery of the species in our waters. Intensive beam trawling is one of the main causes of bottom disturbance, which prevents the growth and development of long-lived species such as the European flat oyster. Besides ongoing bottom disturbance, the degradation of gravel reefs is an additional obstacle to the recovery of the European flat oyster. For years, large stones were systematically removed by fishermen, as a result of which the gravel reefs are currently severely depleted and do not provide sufficient substrate for the attachment and growth of this species.
Recovery of oyster reefs
Situation in our North Sea and surrounding waters
Oyster bank restoration is currently the best-developed form of marine nature restoration in the North Sea as several projects are already ongoing in different countries. This is partly due to the Native Oyster Restore Alliance (NORA)which facilitates the exchange of information on projects.
In Belgium, the UNITED2020 project led by UGent (see bibliography) is ongoing, seeking synergies between nature restoration and aquaculture in wind farms. Aquaculture could initially provide oyster larvae that contribute to the restoration of natural oyster reefs. In the longer term, the recovered oyster populations could then again supply oyster larvae to the aquaculture industry.
In our neighbouring countries, the majority of oyster bank restoration projects are in the implementation phase and preparatory studies and laboratory tests are already behind us (see bibliography). The use of Bonamia-free oysters is a requirement for any oyster restoration project taking place in a zone considered Bonamia-free. Additional knowledge about the occurrence or non-occurrence of Bonamia in the Belgian part of the North Sea and how best to deal with it is therefore a plus for starting oyster restoration projects.
Map showing oyster restoration projects in different EU countries ©NORA
Objectives of oyster bank restoration
Main objective:
Sustainable restoration of healthy oyster reefs and associated long-livingfauna. Restore viable populations of the European flat oyster, which are stable or growing and able to reproduce successfully without human intervention.
Key aspects here:
Restoration of oyster reefsis sought at locations where suitable substrate occurs (whether or not after site restoration) and where oyster reefs historically occurred. In order for oyster reefs to be restored, it is essential that the site remains permanently free from ground disturbance.
Recovery of oyster populations can also take place in other locations such as within wind farms or in aquaculture projects. These oyster populations can support the recovery of oyster reefs and can serve as an additional source of spat that contributes to oyster bank recruitment.
Intermediate objectives:
Substantially reduce pressure from bottom fishing activities on suitable sites for oyster recovery
Reduce/exclude seabed disturbance by taking measures to protect bottom integrity in areas that, possibly after taking certain recovery measures, are suitable for rebuilding oyster populations and/or oyster reefs
See Also{category} for sale in {country}Exclude bottom-fishing activities at site(s) proposed for oyster bank restoration that fall outside the zones where bottom integrity protection measures are envisaged.
Ensure effective implementation of the above measures by providing adequate enforcement.
Increase our knowledge of the European oyster, potentially suitable sites for recovery and potential recovery measures
Conduct research into the presence of Bonamia in the Belgian part of the North Sea
Selection of potentially suitable sites for establishing oyster populations and oyster reefs.
Initiate pilot projects at the selected site(s) to assess their effective suitability for oyster bank/population recovery.
Investigate potential additional supporting measures
Elaborate targeted monitoring plan
Initiate monitoring; conduct T0 monitoring at selected site(s)
Restoration of oyster populations and oyster reefsand associated fauna
Given that the European oyster is an important core species within our waters and has virtually disappeared, restoration of populations of the European oyster is considered a priority. Consequently, in the short term, the focus is on active recovery of European oyster reefs - and populations. Passive recovery, the cessation of local pernicious activities with the aim of regaining a balanced ecosystem, will certainly also be important for the recovery of the oyster albeit rather in the longer term.
Search for suitable sources of spat of the European oyster. Here, it is important to find out whether it is necessary to build a hatchery and/or to join initiatives in neighbouring countries. In addition, ways in which spat from aquaculture projects can make possible contributions will be examined.
Construction and/or expansion of the oyster reefs/ oyster populations if the pilot projects show that the location(s) are also effectively suitable
Monitor oyster populations/ oyster reefs and associated fauna.
Provide additional supporting measures so that the oyster population/bank functions autonomously.
Raise awareness among stakeholders and the wider public about the importance of oyster bank rebuilding
Flowchart oyster bank restoration
A number of pilot projects on oyster bank recovery have already been carried out in the Netherlands and Germany. Based on these, a protocol was written out with the different steps to achieve successful oyster bank recovery (see bibliography). A project running over a time span of at least 10 years is recommended. Four phases can be distinguished here: research and project start-up (year 0-1), test phase, (years 1-3), expansion phase (years 3-6) and development phase (years 6-10). These phases are also shown in the diagram below, indicating whether the phase mainly involves preparation, implementation, monitoring or after-care .
Phase 1. Research and start-up of the project (year 0-1)
A first important step is to determine suitable sites for oyster bed restoration. Conditions must be suitable for the survival and survival of the oyster. In the case of the European oyster, there are several external factors that determine the suitability of a particular site for the species: no to very little bottom disturbance, sediment sufficiently coarse, sufficient oxygen available, sufficient stability of the seabed, limited currents so that larvae do not drift too far away from the oyster bed and sufficient primary production (see bibliography).
Research into possible source populations is also essential. In an ideal scenario, a Belgian oyster bed project increases connectivity between existing oyster beds in the English Channel and in Dutch waters. It should therefore be examined whether, based on the locations of existing/future oyster beds and the prevailing currents, there is a possibility of larval exchange between the existing and the new oyster beds to be established (see bibliography).
Based on previous research, a clear roadmap should be drawn up that further describes the implementation, monitoring and any aftercare required in the subsequent phases of the project. If it should turn out that additional restoration measures are needed to make the site suitable for oyster bed restoration (see A. Restoration of gravel beds), this should be carried out before phase 2, the testing phase, can be started.
Well-developed T0 monitoring is essential in this phase and should be done before the test population is started.
Phase 2. Test phase (years 1-3)
During the test phase, the suitable substrate and a test population are applied in the test site. During the test phase, monitoring is very important and several issues need to be followed up, such as environmental parameters, survival, growth and reproduction of the oysters present. The presence of additional benefits resulting from the oyster restoration, such as additional diversity, local decrease in turbidity (water turbidity), carbon sequestration (removal of carbon from the atmosphere and its storage) in shell and sediment, are also issues that definitely deserve due attention. At the end of the test phase, a decision will be made whether to continue working at the chosen site or not.
Phase 3. Expansion phase(year3-6)
If it is decided to continue with the test site, the actual oyster bed restoration can start by applying more oysters, oyster spat and/or substrate. This phase can focus more on recruitment (increase in number of individuals), spatial distribution around the test site and biodiversity development, and monitoring of any additional benefits as a result of oyster restoration is also important.
Phase 4. Development phase (years 6-10)
During this phase, expansion measures such as spat and substrate inputs can be gradually phased out to verify that the oyster bank is functioning autonomously. Monitoring of environmental variables, recruitment, biodiversity and other benefits should continue. Measures to promote biodiversity can be considered at this stage if key species, which should naturally occur on hard substrate, are missing.
