German Federal Agency for Nature Conservation (BfN)


Hope for the European flat oyster

The European flat oyster is a key species that performs special functions in the marine ecosystem and provides a variety of important ecological services. The oysters form biodiverse, biogenic reefs. Thanks to the large volumes of water they filter, they also significantly improve water quality. A single oyster can filter up to 240 litres of seawater a day.

The European flat oyster was once widespread in the German North Sea. Oyster beds covered many thousands of square kilometres, from the deep tideways of the Wadden Sea, out beyond Heligoland and far into the German Bight.

People have harvested the European flat oyster for centuries. But it was ongoing, intensive harvesting from around 1850 that caused natural oyster populations to collapse across Europe during the 20th century. Large individuals especially were harvested in great numbers for the table. The former reef structures disappeared.

From about the mid-1960s, the non-native Pacific oyster – an escapee from marine oyster farms – has spread along the German coasts. This rapidly reproducing, invasive alien species mostly forms reefs in the tidal zone.

By contrast, the European flat oyster is classed as extinct in the German North Sea, with only scarce remnants of the once large populations to be found. As a result, the species is on the Oslo-Paris Commission’s list of threatened species and habitats. It is also protected under the EU Habitats Directive. Outside of German marine waters, the last vestiges of oyster beds are now the focus of European nature conservation efforts because of their high ecological value. The Marine Nature Conservation Directorate of the German Federal Agency for Nature Conservation (BfN) has also long been involved in the conservation and research into the European flat oyster.

Henning von Nordheim (Statement 1)

We have discovered from historical analysis that, 150 years ago off the German and Dutch North Sea coast, there was a huge oyster bank consisting of the European flat oyster that must have had an enormous effect on the entire North Sea marine ecosystem. We want to try, in part, to restore the functional capacity of that oyster bank by carrying out this reintroduction project in selected sites, mainly in German marine protected areas such as the Borkum Reef Ground. By doing this, we expect to have a very positive effect on the biodiversity of invertebrate organisms on the seabed and most of all on fish stocks and fish production, and last but not least, to bring back in part what effectively used to be a gigantic marine biofiltration system.

In 2016, the Federal Agency for Nature Conservation and the Alfred Wegener Institute launched a major testing and development project called RESTORE to lay key groundwork for restoring stocks of the European flat oyster in the German North Sea.

The team first found areas of the German Bight where there are historical records of oysters and that are suitable for recolonisation. A basic requirement for successful recolonisation is a future ban on all activities that modify the seabed, such as bottom trawling. Suitable areas include marine protected areas and the margins of offshore wind farms, where fishing is not allowed.

Once the necessary permits had been granted, and following a Habitats Directive assessment, work was able to start. Using old oyster shells, the researchers first tried out different techniques for resettlement.

Cage structures and retaining systems were tested in the no-fishing zone around a wind farm off Heligoland. The research also covered the hydrological conditions and the colonisation of old oyster shells with other fauna and flora species, all in preparation for the later resettlement trials with small seed oysters.

Bernadette Pogoda (Statement 1)

In principle, we face two main challenges. The one is more of a technical nature: that of obtaining sufficient quantities of seed oyster material from European breeders. The other is plain and simple: the practical constraints of working in offshore areas of the German Bight. Out there we are reliant on good weather on the North Sea. Working with boats, equipment and divers is quite a challenge – to get all the trials and experimental set-ups running just as we planned.

The preparatory work also includes laboratory experiments. Here, the researchers’ task is to analyse the most suitable settling substrate for oyster larvae. Larvae raised in laboratory conditions were provided with a selection of substrates. Oysters prefer to settle on shell material of their own kind and attach themselves to it with a special sort of cement. But they also grow on stones and on shells of other species. It is now a matter of finding out the optimum material for recolonisation at the selected sites in the wild. In some cases, existing seabed material may have to be supplemented with oyster shells or other settling substrate. Using a microscope, the experts count the tiny oysters on the various materials and compare the results.

Bernadette Pogoda (Statement Ziele)

In the RESTORE project, we are doing key preparatory work for an upcoming large-scale restoration programme for the European flat oyster in the German North Sea. And we are now looking at the legal framework, choice of location, environmental factors at the various sites and, as part of the field experiments, the oysters’ growth rates and state of health, in order to ascertain whether they are still well adapted to the habitat here in the North Sea.

Following strict guidelines, seed oysters only a few millimetres across were selected from the few remaining farms that produce European flat oysters. These tiny oysters are kept, reared, fed and regularly examined at the Alfred Wegener Institute on Heligoland. The observations also give an insight into the mortality rate, which can be quite high in this age group. The young seed oysters are reared for several months up to different size classes. From sizes of about 2 millimetres upwards, they are ready for use in field experiments.

These have been underway since May 2017 in the no-fishing zone around the selected offshore wind farm near Heligoland. Hung in special baskets, net bags with seed oysters were deployed at depths of around 25 metres.

Despite high early mortality, initial successes were already evident after just three months. The surviving oysters had grown from 2 millimetres to about 2 centimetres and after a further month’s stay in the North Sea to about 4 centimetres.

This is a major first success for the German RESTORE project, which is part of a large international network of projects for the resettlement and reintroduction of the European flat oyster.

Scientists from various institutions, universities and agencies are now working in very close collaboration throughout Europe. One of the outcomes is the Native Oyster Restoration Alliance (NORA) launched in Berlin in November 2017 at a workshop initiated by the Federal Agency for Nature Conservation. The network brings together nature conservation agencies, researchers, conservation organisations and oyster farmers.

The Alliance’s long-term goal is to re-establish the native European flat oyster as a key species in the North Sea and adjacent European seas and to restore its biodiverse reef structures.

Henning von Nordheim (Statement 2)

The European flat oyster, as a biogenic reef-building organism, is a strictly protected habitat type under EU nature directives. This means it ought to be a focus of conservation efforts for all Member States of the European Union, and it has indeed attracted heightened attention in recent times. We see cooperation with neighbouring countries, which is currently coordinated via the NORA project, as an ideal starting point for protecting the remaining populations of oysters in Europe, and also for rebuilding oyster populations, for example by the exchange of seed material. There is also a requirement under the Oslo-Paris Convention for the Protection of the Marine Environment of the North-East Atlantic, which likewise has a focus on the oyster. Backed up by national projects like RESTORE, NORA is an ideal forum for driving ahead this sort of European cooperation, with the ultimate aim of restoring populations throughout Europe.