Nature development in the North Sea

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 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 flat 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 native 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 the 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 new concepts

In the future, the artificial reefs may become so productive that they can yield oysters, fish or lobster harvests. “Then 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 this so-called 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 new concepts. We also have experimental setups closer to the coast, including in the Zeeland waters. There are racks there that we fish up every year to see how fast the oysters have grown.”

More information:

Read the more extensive interview with Oscar Bos about nature development in wind farms.