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On a square kilometre, researchers give sea life a helping hand

The Hague, 5 March 2020

What are we looking at on the laptop of researcher Christiaan van Sluis? Are the moving images a glimpse into an intestinal examination? Fresh impressions of the landing on Mars? Neither: on board the refurbished Scheveningen-based trawler Aquila, the marine biologist shows recordings of the North Sea seabed that he has just taken with an underwater camera, around 12 kilometres off the coast of Scheveningen.

The image: while clouds of tiny organisms seem to move like constellations through the universe, the camera pans over an empty landscape full of ridges and craters. They seem bare, unpopulated areas, but the biologist’s experienced eye detects some life forms. ‘Look, a brittle star,’ Van Sluis points excitedly to his laptop, which reveals a grainy image of a faint ridge in the sand. Unusual? Not really, it’s a common species for the North Sea. Not a crab or fish to be seen, but from a single shadow in the sand, Van Sluis recognises a piece of peat, a worm or a starfish.

It’s not much. Which is exactly what Van Sluis expected and even hoped for. It’s also the reason why we are here with a team of innovative researchers and likeminded people looking for traces of life. The first measurement provides the proof: the seabed here consists of fine to medium-coarse sand. Noted.

‘Here’ is the seabed of a ‘sea farm’, a test site covering a square kilometre where they are doing tests to restore the biodiversity of the seabed. Which is necessary, because with its use of increasingly large and heavy dragnets, fishing is particularly harming life on the seabed.

The fact that today’s shots, taken with a simple, DIY installation made of grey PVC tubes, a GoPro camera and a strong torch, reveal the emptiness of the seabed is therefore a bonus. Eventually, it should be possible to observe a difference with an improvement in sea life, the main aim of the researchers behind this operation.

Today – the first spring day of February, the sun was veiled by Sahara sand – eight representatives from organisations are on board. The idealistic organisation North Sea Farmers rents the test site, an invisibly cordoned off piece of sea totalling 6 square kilometres within sight of the coast, on the left an oil and gas platform, and on the right numerous moored tankers awaiting a new cargo or speculating on the oil prices in the port of Rotterdam.

The layman sees nothing of all this, but this is a unique location: the North Sea is an area in which every square centimetre is used at least threefold, for navigation, fishing and cabling. Except for this test site, cordoned off with buoys. The plot is divided into six equal pieces, on which different parties test various sustainable innovations, including mussel farming and installing solar panels at sea. North Sea Farmers itself is experimenting with growing seaweed, a sustainable product which they feel has great potential for the future. The ambition is to create 400 square kilometres of seaweed farming in the Netherlands sea.

Wind farm

Thanks to the National Postcode Lottery, which donated 8.5 million euros from the so-called Droomfonds, for this initiative Stichting Natuur & Milieu and De Noordzee are working together in De Rijke Noordzee, which aims to improve nature around wind farms. This spot offers a wonderful opportunity for experiments, because it is difficult working in and around wind farms. Unlike in the rest of the sea, the seabed is left undisturbed here – precisely the reason why these places offer opportunities for nature to recover without the usual disruption from shipping and fishing. An additional advantage of this test site is that it is closer to the coast, so easier to access than the wind farms which are considerably further away.

‘Besides opportunities, wind farms certainly involve risks,’ agrees Erwin Coolen, programme director of De Rijke Noordzee. ‘Take collisions with birds and bats, for example. We hope to transform those risks into opportunities for nature, something we are working on with De Rijke Noordzee.’

To promote that development, preferably worldwide, Coolen’s organisation is experimenting at six sites on the Dutch coast with nature development at wind farms. The hope is that these parks can become the nurseries for underwater life. ‘Nature here is getting a boost.’

Artificial reefs

How? For example, by creating artificial reefs on the seabed. Because natural reefs have largely disappeared from the bed of the North Sea, the artificial reefs must attract mussels and other life that needs to attach itself to the structures. Sometimes, that seems simple: hang a rope in the sea or in a port and there’s a good chance that mussels will soon grow on it. The principle of these artificial reefs is the same. On the seabed, they must be the first step for the formation of new natural reefs. If this proves successful, this idea will be implemented more widely in wind farms, more of which are likely to appear in the coming years.

Today, the big day, various types of artificial reefs will be placed on the seabed. A measuring buoy placed at the same time will continuously generate data about the water temperature, salt levels and acidity. The reefs will be placed on the seabed from the Rijksrederij’s MS Rotterdam using big equipment at a depth of around 18 metres – the North Sea is not much deeper than that here. These are big concrete blocks, several metres long and wide, with large holes, each connected to their own orange buoy to mark them.

On and around the concrete blocks, structures have been made to which life will hopefully attach itself. The holes in a block are filled with jute bags containing scallop shells. The jute will soon rot, while the grit from the shells will spread around the reef and form a source of food. On one of the oyster cages, the researchers have attached thick branches and twigs of pear wood. Pear wood? That doesn’t grow in the sea? No, but it’s a strong type of wood that doesn’t rot quickly in water. Above all, it’s an experiment, like everything in this project. The first tests with hard materials for the structures were done in labs. Different species must be able to attach to them.

With all the structures, among others the researchers hope to see the ordinary mussel. It would be nice to see squid-like creatures or even sharks and rays laying their eggs there, the researchers dream out loud. They also expect crabs, coalfish and sea anemone to find and attach themselves to the reefs.

Whether this will happen is the question. Trial and error are the key ingredients of experiments. That was evident in July 2019, for example, after a test involving the restocking of flat oysters to the Luchterduinen wind farm, off the coast of IJmuiden. Eight months later, 85 percent of the oysters had died. ‘After a month, the basket containing the oysters had tilted in the sand,’ the researchers now know. This starved them of oxygen, causing them to die.

This time, there will be no restocking of oysters, although there will be two oyster cages on the seabed. This is not just related to the previous disappointing experience. To restock oysters in the sea, a permit is required, which takes around six months to obtain. The researchers didn’t want to spend that time twiddling their thumbs.

The new oyster cages were designed by the NIOZ, the Royal Netherlands Institute for Sea Research. The concrete artificial reefs were designed by the British company ARC Marine, for which it won the Offshore Wind Innovation Challenge in 2019.

At this stage, the main thing is to ensure that they are stable enough and don’t sink into the sand, as previously.

Facts, figures, data

The plan is for the reefs to remain in place for three years. Every six months, the researchers will inspect them with cameras or divers to chart how the processes are going. Because if anything is needed, it’s facts, figures, data. You might not expect it, but this is what the researchers have found: life on the seabed of the North Sea is almost unexplored territory. ‘We know a lot, but there’s also a lot we don’t know about the life under water,’ says ecologist Van Sluis. One of the unsolved puzzles is the question why the Japanese oyster thrives in almost all circumstances, but the native flat oyster doesn’t. The different structures installed on the artificial reefs will provide clarity about what can happen when you leave the seabed undisturbed and let nature do its thing.

The flat oyster is a useful creature: it filters the water, builds reefs, recycles nutrients, stores carbon, and its presence attracts other animal species. Complication: in the wild, the flat oyster is increasingly being targeted by the parasite Bonamia, which arrived in Europe at the end of the 1970s via oyster transport from California. The parasite causes infections and can be fatal. This is one of the reasons why the flat oyster has largely disappeared in the Netherlands. Restocking flat oysters, with the help of cages and artificial reefs, could help it recover.

At the end of the afternoon, the job is done. The crew looks with satisfaction at the perfectly straight line between the buoys which are anchoring the different test reefs. Bottles of seawater go in the cooler bag: DNA from the samples should reveal whether and what life exists in this piece of the North Sea.

Fortunately, they still have the underwater images, which are viewed while the Aquila sets off on the return journey to the coast of Scheveningen. In six months, the team will go back to sea to check what has happened in the meantime.


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