Sustainable mussel farming

Photo: Karin Troost


The Wadden Sea is a unique ecosystem. Simultaneously, it’s the mussel farmers’ fishing pond for mussel seed (known as spat), which they use in their mussel plots that are also located in the Wadden Sea. Research by WUR has shown that this need not harm biodiversity if done properly.

Wageningen Marine Research has been researching wild mussel beds, ecology and the effects of mussel culture on nature in the Wadden Sea for 25 years. That expertise is being made available to the wider world in the Tailored Knowledge project ‘Shellfish: sustainable and healthy’. Juvenile mussels (the spat) are essential for mussel growers. Without that seed, there would be no mussels. Mussel seed is a somewhat confusing name. Wild mussels shoot out sperm and ova into the water, and their fertilisation results in tiny mussels a couple of millimetres in length, which look like seeds. The mussel seed floats around the Wadden Sea in enormous quantities in spring. Then they settle in certain spots on the bed of the Wadden Sea during, known as the ‘spatfall’. Mussel farmers are allowed to collect the mussel spat during a limited period, which they then cultivate in plots to produce mussels suited for consumption.

Permit to collect spat

Each year, the WUR researchers check how much mussel seed has fallen and where. Then the mussel farmers apply for a permit to fish a certain quantity of spat. A key agreement here is that the total biomass of mussels must remain the same as it would be in a situation with no spat fishing or mussel cultivation. Additionally, certain areas are designated ‘closed areas’ where no fishing is allowed. That protects wild mussel beds and the supply of food for birds such as the common eider, making the agreement absolutely vital.

Mussel seed at the bottom of the sea. Photo: Karin Troost

The WUR researchers know a lot about mussels, but not everything. It is still a mystery, for example, why there is a lot of mussel spat in the Wadden Sea one year and hardly any the next. “You have years of plenty and lean years, but we haven’t yet been able to find the mechanism behind this,” says Nathalie Steins, who heads Wageningen Marine Research’s Tailored Knowledge project. There were several years in the late 1980s with not much spatfall.

Overfishing = not enough food for birds

Steins: “The mussel farmers needed seed to grow their mussels but if there isn’t much seed, you start looking at all the places that still have some. The mussel farmers hired cockle ships. They have a shallow draught and when tides are high, they can sail over mussel beds that are exposed at low tide. The farmers fished almost all the available mussel spat. The few cockles that were around were all fished by the cockle fishermen as well. The combined effect of this was widespread mortality among birds such as the common eider and the oystercatcher due to a lack of food.”

Mussel fishermen from Zeeland transport mussel larvae from the Wadden Sea to their own mussel plots, where they grow into adult mussels. WUR researcher Henrice Jansen is investigating whether mussel farming and nature can go together. And they do: the biodiversity of a mussel plot is comparable to that of a wild mussel bed.

The mussel and cockle fishers came in for a great deal of criticism for what they had done. Nature organisations accused the fishermen of having overfished the Wadden Sea until there was nothing left. In response, the sector voluntarily made agreements on the availability of food for birds and marked off areas where fishing for mussel seed would no longer be permitted to allow the wild mussel beds recover. Later on, these and other measures became part of government policy. Ten years ago, an agreement was also made to develop new methods for collecting mussel seed.

Fishing for mussel seed is not allowed in certain areas. That protects the wild mussel beds

New method for catching spat

Until the early 2000s, mussel spat was caught using a steel frame with a shallow net that was dragged across the mussel seed bed or mussel farm. A different method has become popular in the last 12 years or so. Known as the Seed Mussel Collector (SMC), it consists of poles in the sea with connecting ropes that the floating mussel larvae attach onto. About half of the spat is now caught using these SMCs. The idea is that this method is more environmentally friendly because it does not stir up the seabed or disrupt the biodiversity there. But as Nathalie Steins points out, “When you set up SMCs, you are also introducing structures in the water that are not biodegradable: nylon, plastic and metal. There is always a small risk of parts of the SMCs coming loose due to strong currents or storms.”

A cockle ship with a Seed Mussel Collector. Photo: Lando Nieuwenhuize

Floating mussel larvae attaching onto the ropes. Photo: Karin Troost

With their irregular shape, wild mussel beds not only supply mussel seed, but also provide an ideal spot for crabs, starfish, lugworms, fish, lobsters, barnacles, sea anemones and so forth. Fortunately, the same applies to the mussel farm sites. As many as 102 different species have been counted in the mussel farm sites compared with 84 on the wild mussel beds. It should, however, be noted that the farms are mainly in areas where the salinity is higher, and the saltier the water, the greater the biodiversity. Mussel farmers also try to keep their sites clear of starfish, which eat mussels and other species that live in their vicinity.

Shellfish farming is a completely natural process without any additives, fertilisers or pesticides

Mussels are the grazers of the Wadden Sea. But does the Wadden Sea have enough food for them all, given that wild shellfish and cultivated mussels are eating from the same trough? “Mussels and other shellfish filter the Wadden Sea water like mad and eat the microalgae they get out of it,” says Steins. “As you can imagine, the more shellfish there are, the less food there is because the algae can’t reproduce fast enough. However, research by Wageningen shows that the supply of food for shellfish, including the cultured mussels, is not at risk at the moment.”

Microalgae and microplastics

Mussels filter more than just microalgae out of the water; microplastics have also been found in the gastro-intestinal tracts of mussels. Steins: “It is known that mussels can filter microparticles measuring between 5 and 40 micrometres. Some of this may end up in the gastrointestinal tract. If you eat mussels as part of a normal menu, there is no reason to assume this would lead to large-scale exposure to plastic particles, certainly not compared with other sources such as household dust.”

The Tailored Knowledge project ‘Shellfish: sustainable and healthy’ is making knowledge that has been acquired over the past few decades available to mussel farmers. Steins: “These days, you must be able to show that your business practices are socially responsible. That means you must have answers to questions such as ‘Will there still be enough mussels left in the Wadden Sea, and what effect does seawater pollution have when eating mussels?’”

Fishing for mussel seed in the Wadden Sea. Photo: Karin Troost

Steins says mussel farming, when properly regulated, and nature conservation can go well together. “When I talk to colleagues in other countries, farming and fishing for shellfish are seen there mainly in positive terms as things that are good for biodiversity, the ecosystem and water quality. They are often surprised that we have a debate in the Netherlands about whether a nature area like the Wadden Sea should also be used for producing shellfish for human consumption.”


“If you want to eat animal products with a minimal environmental burden, you should eat shellfish,” says Steins. A review of nearly 200 studies on the ecological footprint of animal food products from fishing, aquaculture and livestock farming shows that shellfish score best. According to Steins, that is because shellfish farming is a completely natural process without any additives, fertilisers or pesticides. “All you need to do is catch and harvest the spat. So if you want to be ecologically in tune with the times, you should eat mussels.”

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