Artificial reefs for oysters and other marine life

Until about one century ago, the Dutch North Sea harboured extensive oyster beds. They were made up of many generations of oyster shells, which formed a substrate for the growth of new oysters. “These reefs also provided a habitat for a variety other marine animals”, says Oscar Bos of Wageningen Marine Research, “including fish, anemones, tube worms and various types of shellfish.”

But due to overfishing in the 19th and early 20th centuries, the European flat oysters, also known as flat oysters, almost completely disappeared from Dutch waters. A viral disease in the 1950s was the final blow for the remaining oyster beds – and for the associated biodiversity. From the 1970s, an exotic oyster species took over: the Pacific oyster. Native oysters are now mostly restricted to oyster farms in the province of Zeeland.

Extra biodiversity

“The Pacific oyster also forms reefs, but only in the shallower parts of the North Sea”, says Bos. “That is why initiatives have been launched in the Netherlands and neighbouring countries to return the European flat oyster to the North Sea. This is only possible, however, in locations without trawler fisheries.”

For this reason, offshore wind farms constitute a potential new oyster habitat. However, oysters will not easily settle in the absence of solid structures on the seabed. It is difficult for them to anchor in the sandy North Sea bottom. “But in wind farms, stony materials are usually deposited around the bases of the turbines”, says Bos, “to protect them against the abrasive action of sea currents. We are investigating under which circumstances these stones are suitable for flat oysters – and whether you could get extra biodiversity if you create cavities for animals such as fish, lobsters and crabs.”

Testing concepts

Artificial reefs in Denmark are known to attract all kinds of marine life. “We want to see whether this is also the case in the Netherlands”, says Bos. “Obviously, the main purpose of wind farms is energy generation. But the government is also encouraging nature development in wind farms. Making a positive contribution to the ecosystem even plays a role in the tendering of new parks. Wind farm developers can score points with plans to increase biodiversity.”

In the future, artificial reefs may become so productive that oysters, fish or lobsters could be harvested. “The reefs could become commercially interesting”, says Bos. “But first we would like to find out what works best, and which species can benefit from what we call nature-inclusive building.” Wageningen Marine Research is now developing concepts for this, in collaboration with Bureau Waardenburg, Deltares, the Rijke Noordzee Foundation, the World Wildlife Fund, the Ark Foundation and other partners. “We are already testing the first ideas.”

Natural and artificial reefs

Restoring native oyster beds has been on the radar of researchers for some time. “A few years ago, adult oysters were deposited in the Gemini Wind Farm, north of the island of Schiermonnikoog”, says Bos, “and at the Borkum Reef Grounds, an area bordering German waters. Here, glaciers left behind gravel and rocks after the ice ages. This material is very attractive to all kinds of shells, tube worms, burrowing crustaceans and other sea creatures. Flat oysters turn out to be reproducing in both places: we find oyster larvae there in the summer.”

Doing research in such places is difficult, due to practical challenges. “Strict safety measures apply in wind farms, and their remoteness excludes certain types of research”, says Bos. “That’s why we also have experimental setups closer to the coast. There we have racks that we hoist up every year to see how fast the oysters have grown.”

Possibilities

The experiments are still small-scale. For the time being, introducing flat oysters on a larger scale is not yet realistic, Bos emphasizes, because the ‘source material’ is still very scarce. “My colleague Pauline Kamermans, our oyster expert, is studying the potential of oyster hatcheries in this regard,” says Bos, “and we are investigating which offshore locations could be suitable. We also use cameras and DNA techniques to see which fish and other animals live around the stones on the seabed.”

Bos laughs: “Yes, we are still in the phase of trying out as much as we can. Creating artificial reefs in offshore wind farms may sound far-fetched”, he notes. “But I see it this way: offshore wind is inevitable in our energy transition. Our government has made it a priority. Then we’d better get the most out of it – not just energy, but biodiversity as well. And maybe even food, in the long run.”