ECO-FRIEND: Restoring oyster reefs in the North Sea


Recreating hotspots for life

Until the early 20th century, flat oyster beds were a widespread biodiverse habitat in the North Sea, stretching over thousands of square kilometres. Reefs formed by the oysters are keystone habitats that create favourable conditions for many other species and have an important positive impact on the North Sea ecosystem. However, at the end of the 19th century, oysters were discovered as a cheap food source for relatively poor industry workers. As a result of overfishing, the oyster beds rapidly declined and became extinct. The ECO-FRIEND project aims to restore these oyster beds.

The ECO-FRIEND project (2019-2023) was set up by four parties, with oyster expertise ranging from oyster biology and modelling the environment to developing innovative monitoring methods. The project aimed to provide insights that help the restoration of oyster reefs. The project goals include:

  • The monitoring of pilots with the restoration of European flat oyster beds in offshore wind farms
  • The development of new monitoring methods to assess the effectiveness of these pilots, reducing cost and increasing scientific output
  • Understanding the environmental background of the pilots by intensive monitoring and modelling of the surrounding parameters (e.g. temperature, salinity, food availability, waves and currents)
  • Analyse, discuss and publish the findings in scientific and industrial literature
  • Provide advice on the eco-friendly design of future wind farms.

Focus on simple and cheaper ways of research

Hein Sas (Sas Consultancy) is one of the ‘founding fathers’ of oyster restoration in the Netherlands and explains the background of the project: “Restoration is necessary because oyster beds are an important link in the ecosystem. They filter the water and provide food, shelter and attachment opportunities for a wide range of organisms. The current focus of oyster restoration efforts is in offshore wind farms because they provide a protected environment inaccessible to bottom trawlers. However, in offshore conditions, the monitoring of oyster restoration projects can be extremely expensive and time-consuming. Therefore, in the ECO-FRIEND project, we focussed on the use and innovation of affordable monitoring methods.”

Lightweight techniques reduce costs

In earlier projects, to assess the growth and survival of adult oysters, ‘tables’ with oysters inside have been deployed by large cranes at very expensive offshore construction vessels. In ECO-FRIEND, the researchers focussed on lightweight techniques that can be deployed by hand from smaller ships like crew transfer vessels. e.g. small crew transfer vessels.

Researcher Oscar Bos, marine ecologist at Wageningen Marine Research, was involved in the ECO-FRIEND fieldwork: “We monitored pilot oyster restoration sites in the Gemini wind farm, 60 km offshore and at the Borkum Reef Grounds closer to the coast. At both locations, thousands of oysters were put on the seafloor in 2018. The area is located within the historic range of the European flat oyster. We also tested techniques nearshore in the Voordelta area, where a natural oyster reef is present.”

Bos: “To monitor oyster growth at Gemini, we used small cubic cages with baskets with 20 oysters each, but they were too light and were taken away by the currents”. As an innovation, Waardenburg Ecology developed a more stable yet still lightweight device on which baskets with oysters can be placed. It can be easily deployed on the seafloor and picked up by using a small ROV.“ Another technique we tested at the restoration sites was the larval pump to monitor if oysters were indeed reproducing. We took seawater samples from near the seabed at 30 m depth, and sieved hundreds of litres of seawater over a small mesh size, studied the samples under the microscope and with DNA techniques. This proved to work. But we also found larvae at reference sites where no oysters were expected to be present. We hope to solve this puzzle with the Deltares modelling work.“

“Nearshore, we tested the oyster valve monitor, a device which can measure oyster activity. Individual oysters are equipped with electrodes measuring the opening between the two valves. The valve monitor registers whether the shell is closed or open, which relates to daily activity such as feeding or stress. And, of course, when it remains open all the time, the oyster is dead. In that way, we can trace whether oysters were active on the seafloor at a certain time so that we can correlate this with environmental variables.“

Since oysters are expected to attract other organisms, monitoring also focussed on the biodiversity of the fish species present. “Now used in many other projects, eDNA sampling of biodiversity was relatively new when we started the project.” Bos explains. “This technique detects DNA present in the water originating from, for example, lost skin particles of fish. The eDNA results largely matched those of our bait camera video and showed the presence of fish like plaice, gobies, sand eel and horse mackerel. Certain species showed in the e-DNA analysis but were missed by the cameras so e-DNA proved to be an important addition to our monitoring techniques. Now the technique is used by default in oyster restoration studies.”

Movement of oysters on the seabed?

It is important that oysters are deployed at carefully chosen restoration sites and are not washed away by waves and currents. At the Borkum Reef oyster restoration site, the thousands of adult oysters that had been put on the seafloor in 2018 could not be found. The question was: where did they go? This led to an interesting experiment in the ECO-FRIEND project. Luca van Duren (Deltares) explains: “In our flume tank, we placed oysters of different sizes and weight classes on the sandy floor and subjected them to waves of different heights. Meanwhile, we filmed the oysters both underwater and from above to see how they would behave under the different hydrodynamic conditions. The data that we obtained from these experiments are now used in numerical models to assess the stability of deployed oysters and oyster beds. We can do this not only for the Borkum Reef area but also for other parts of the North Sea.”

Predicting oyster reproduction and larval spread

The Deltares 3D model is a powerful tool that can increase restoration success, since it helps to develop optimal restoration methods for specific conditions in different areas. Van Duren continues: “In combination with knowledge on the biology of oysters, we can now predict for any location when in summer oysters will reproduce, and where larvae are transported to, based on the local temperatures and currents.” The existing model was set up using large-scale datasets but needs local data to validate it for specific sites. Bos: “In our case, we have even been able to use an unexpected source: harbour seals with data loggers. The seals had been visiting our oyster restoration sites. So, we used their logged temperature and depth data! In general, we strongly recommend for any restoration project to use environmental data loggers which will help to understand and predict the success of the restoration efforts.”

Follow-up project awarded with a new grant

Future monitoring missions will focus on oyster growth, survival and reproduction. The TKI Agri & Food programme has allocated budget for a follow-up project: Better Oyster Outplacing & Seeding Technique (BOOST). Bos is satisfied: "The BOOST project focusses on installing so-called ‘reef tiles’. The idea is to let oyster larvae settle on the tiles in a laboratory setting and then place the tiles on a restoration site. Because of their shape, these tiles are not so easily transported away by the currents. They are small and easy to produce and transport. They are also completely degradable in nature and disintegrate into natural components after a couple of years.”

The expanded knowledge base on flat oyster restoration developed in the ECO-FRIEND project has already been used in more recent oyster restoration initiatives in the Dutch North Sea. The knowledge also was made available internationally through the European Native Oyster Restoration Alliance (NORA). The results are also valuable for new offshore wind energy tendering processes in the Netherlands, in which measures to improve the ecology play an important role. This way, the Dutch government tries to strengthen the North Sea ecosystem to meet international biodiversity objectives.

Interested in more information? You can follow the links below:

European Native Oyster Habitat Restoration and Monitoring Handbook

Project Database TKI Wind op Zee