Swimming through the Afsluitdijk

Published on: 22 April 2024, 11:34 hrs - Latest update: 14 May 2024, 08:59 hrs

The construction of the Afsluitdijk has benefited the Netherlands in many ways, but for the fish population in our country, it was disastrous. With the Fish Migration River, among others Wouter van der Heij and Bart Noordman are attempting to reverse this ‘ecological disaster’.

‘It is fabulous to see the results that ecologists and engineers can achieve together.’

There was a time when our inland waterways were populated with hundreds of thousands of salmon. When the smelt was so plentiful that farmers used them to fertilise their fields. And when we still had our own Dutch herring population.

This was the time of the Zuiderzee.

‘The greatest ecological disaster’

‘Let there be no mistake: the Afsluitdijk is a project of which the Netherlands can be duly proud. But from an ecological perspective I often refer to it as the greatest disaster we have ever experienced,’ sighed Wouter van der Heij, marine ecologist and deputy director of the Wadden Association.

And not without reason: the Zuiderzee was a transition area between freshwater and seawater and for certain migratory fish species it was the ideal spawning ground. The Afsluitdijk sliced this area of brackish water in two, creating a barrier between the seawater of the Wadden Sea and the freshwater of the IJsselmeer. ‘And as a consequence, the fish were unable to complete their journey, and disappeared.’ 

When Rijkswaterstaat announced plans to reinforce the Afsluitdijk in around 2010, and invited regional players to contribute their ideas, Wouter recognised this as a pivotal moment. ‘In our opinion, merely strengthening the dyke was not good enough. There also had to be attention for ecological recovery. And that is how we came up with the idea of creating a gap in the dyke.’

The Fish Migration River

The idea was adopted, the funding organised and Wouter and his colleagues were soon able to start further elaborating the plans. To make sure that the entire project was carried out responsibly - because to be perfectly honest, making a hole in a dyke does not sound the safest of activities - Bart Noordman, a consultant specialising in hydraulic engineering structures at Rijkswaterstaat, and his team joined the project group 6 years ago.

‘Safeguarding our water safety is has always been the highest priority’, explained Bart. ‘But attention for other objectives, such as ecology, freshwater retention, water discharge and securing the passage for shipping, have become integral to our work.’

In practice it is often extremely difficult to combine all of these functions. For example, it is not possible to allow fish to pass when a barrier is closed as a precaution, at high tide. Bart continued, ‘The result is an area of tension in which you must constantly seek the best of both worlds.’

Fortunately, in this case, cooperation was excellent and all parties were open to each other’s ideas. Eventually this led to a carefully conceived balance. A ‘river’ that meanders over several kilometres, cuts clean through the Afsluitdijk, and reunites freshwater with seawater: the Fish Migration River.

From seawater to freshwater, from low to high tide

In principle, the Fish Migration River was designed for the 12 fish species which after 90 years still swim in our waters, albeit in massively reduced numbers. They are all very different: from the stickleback, a tiny fish measuring just 3.5 cm, through to the sturgeon that can grow up to 3.5 metres in length. And from the salmon, which can easily overcome the strongest of currents, through to elver (glass eels) that must allow themselves to be borne along by the water. In the near future, all of these species will be able to make the crossing, 24 hours a day.

‘The engine that drives the system is the tide,’ explained Wouter. ‘At low tide, when the water in the Wadden Sea is lower than in the IJsselmeer, freshwater pours out of the IJsselmeer through the gap in the Afsluitdijk. This attracts the fish swimming in the Wadden Sea. At high tide, the opposite happens: seawater floods through the same gap in the other direction. Thanks to this current, even the smallest elver can make the crossing through the dyke. And this process takes place throughout the day: the water flows backwards and forwards, leaving the fish free to swim.’

Journey through the Afsluitdijk

Once they enter the gap in the Afsluitdijk, the fish find themselves in an environment with a bottom strewn with large rocks. Here they can take a short rest on their journey. ‘An excellent example of how we together arrived at a balance,’ grinned Bart.

‘From the point of view of water safety, what you want is a smooth and strong bottom profile to avoid the possible risk of scouring and to ensure that the water barrier can be sealed tight. Fish on the other hand, are never happier than with a bottom with height variations. The compromise involved a design with rocks cast into concrete.’

After passing through the gap, on their journey to the IJsselmeer the fish first pass through the water harmonica: a meandering ‘river’ with a length of 4 km. At least that is the plan for the future, because it is the only part of the Fish Migration River that has not yet been completed.

This ‘river’ has been designed in such a way that the salt content gradually decreases to ensure that the IJsselmeer remains a freshwater body, at all times,’ explained Bart. ‘It also means that the fish can get used to the transition to freshwater,’ added Wouter, ‘in the most natural way possible.’

Unique innovation

If everything goes according to plan, can we expect our rivers to be overflowing with salmon, like they were in the past, once the Fish Migration River is completed in 2025?

'I wouldn’t like to hazard a guess,’ replied Wouter with a smile. ‘But of course I do hope to see a rise in population numbers. Since 1933, you could say the population has fallen from 100 to just 1; it will be a real step forward if as a result of these measures we are able to once again achieve a score of 5 or 6.’

Bart is also very optimistic about the end result. ‘This is unique. Nowhere else in the world is there anything that is even similar to the Fish Migration River. If it proves effective, it is my fervent hope for the future that it will become an export product.’

In closing, Bart added that this result would never have been achieved without the excellent cooperation. ‘Success or failure depends on a good personal relationship, an open mind and the will to understand each other. These conditions are often lacking, but fortunately they came together here. They form our success factor.’

The initiative for the Fish Migration River was the brainchild of the Wadden Association, It Fryske Gea, Sportvisserij Nederland, NetVISwerk and the Blauwe Hart Natuurlijk coalition.

The project is commissioned by the Province of Fryslân and was made financially possible by the Provinces of Noord-Holland and Fryslân, the Dutch government, the Waddenfonds, LIFE IP Deltanatuur, Natura2000, the Nationale Postcode Loterij and Connecting Europe Facility transport (CEF).

More information Afsluitdijk

For more information about the Fish Migration River, check Vismigratierivier Afsluitdijk or visit