The European Flat oyster, back from (never?) being gone?

Long ago, the Flat oyster was ubiquitous in the southern North Sea. But due to overfishing, pollution and diseases, this native species almost completely disappeared from our country. Today, there are signs that the Flat oyster has begun a cautious recovery. This is demonstrated by finds in unexpected places, such as in ports and offshore wind farms.

Kelle Moreau

The fact that the Flat oyster (Ostrea edulis) has been found again in Belgian waters can certainly be called a sensation. After decades of absence, marine biologists have found living specimens and empty shells of Flat oysters on all kinds of human infrastructure, including scientific instruments. Flat oysters are now also being found in port areas such as Zeebrugge and Ostend.

In addition, offshore structures, such as the steel foundations of offshore wind turbines, provide new habitats for oyster larvae to attach. Beachcombers have found live specimens and fresh shells of Flat oysters washed up on the beaches of the western coastal zone.

Francis Kerckhof and Thomas Kerkhove of the research group ‘Marine Ecology and Management’ (MARECO) of the Institute of Natural Sciences summarize the recent finds in an article in ‘De Strandvlo’, the magazine of the Belgian Beach Working Group (Strandwerkgroep België), and examine possible explanations. What is striking is that the new finds seem to be at least partly of wild origin. A comeback to the rhythm of the sea.

Why are ports and wind farms interesting for oysters?

The modern seaport has surprisingly become a suitable habitat for some species that used to occur in natural reefs. Solid structures such as quay walls, pontoons, mooring posts and shipwrecks provide hard substrates to which oyster larvae can attach. Furthermore, harbours often provide a certain shelter, which facilitates larval settlement. In addition, the water quality has improved considerably in recent decades.

Offshore wind farms are a similar story. Underwater, the foundations transform into artificial reefs that attract life, from starfish and sea squirts to mussels and… Flat oysters. The structures are little disturbed because shipping and fishing are not allowed, while pilot projects focusing on the recovery of Flat oysters ensure more larvae in the water. That turns out to be exactly what a species like Ostrea edulis needs to regain a foothold.

A history of loss

The Flat oyster has a turbulent history in Belgium. It was once ubiquitous in the southern North Sea, and a culinary icon. Until the beginning of the 20th century, oysters were harvested in abundance from natural banks in the North Sea and watered down in so-called oyster pits on the coast, among others in Oostende (the famous Ostendaise). But due to overexploitation, habitat loss, pollution and the creep of diseases and the oyster parasite Bonamia ostreae, the species disappeared from our waters.

In the second half of the 20th century, commercial oyster farming switched to the Japanese oyster (Crassostrea gigas), which is easier to grow and less susceptible to the oyster parasite. As a result, Ostrea edulis also faded further into the background in Belgium.

Why is the return important?

Francis Kerckhof: “The rediscovery of the Flat oyster is not only culturally and historically interesting, it is above all an ecological story. After all, the Flat oyster is an essential part of the marine ecosystem. As a filter feeder, it helps keep the water clear, and its reefs create a habitat for countless other species. Where there are oysters, underwater life flourishes. Moreover, the Flat oyster is a native species, it belongs here by nature and its recovery can contribute to the marine recovery of the North Sea.”

In addition, this comeback is part of a broader European trend. In countries such as the United Kingdom, Germany, the Netherlands and France, reintroduction programs have been started to re-establish the species, and in Belgium there are also pilot projects supported by Belgian and European funding.

And now?

For scientists, this rediscovery is both an opportunity and a challenge. Can we use monitoring and DNA analysis to determine where these oysters come from? Do they come from isolated relict populations, did they come with shipping or ocean currents from locations where restoration projects are underway, or did the oyster larvae come from commercial oyster farming?

At the same time, the new knowledge also raises policy questions. Should we support this natural recolonization? Can ports or offshore installations be consciously designed as oyster-friendly structures? And how do we avoid repeating the same mistakes of the past, such as overexploitation or insufficient protection?

Thomas Kerkhove: “For the time being, the number of Flat oysters is still limited, and there is no talk of large populations yet. But the fact that Ostrea edulis is spontaneously appearing in various places is a hopeful sign. The sea shows us that, if we give it space, recovery is possible. In this context, every find on the beach or on a quay wall is a small reminder of the resilience of nature.”