Sunday, October 17

The Netherlands plans to open one of its dikes to fish migration with a work that recreates a river | Ecology | Climate and Environment

Located in the northern part of the Netherlands, Afsluitdijk, the dike that has protected its inhabitants from the floods of the North Sea since 1932, is one of the prides of national engineering. 32 kilometers long, 100 meters wide and 7.5 meters high above sea level, it also reflects the change that has occurred over time in the search for safety in the face of natural catastrophes. The ecological impact of the effort in the struggle with the sea that has made the Dutch famous is a factor that has been gaining ground over the years. For this reason, at the time of its construction, protection prevailed over the fact that this barrier prevents the migration from the sea of ​​species such as eels, Atlantic salmon or herring to the interior freshwater reservoir thus created, the Lake Ijssel (Ijsselmeer, in Dutch).

Although these migratory fish need both fresh and salty environments to breed and grow, only the best swimmers make it across the dike openings used to release lake water at low tide. Solution? Open a passageway and create docks on both sides so that they can enter and exit without problems. Called The river for fish migration, the project is presented as the largest of its kind on a world scale and aims to repopulate the lake’s fauna by working together with nature. The work is already underway, its opening was scheduled for four years from now, but it is not ruled out that it may work even earlier, in 2023. It will cost 55 million euros.

Once completed, the road that runs over the Afsluitdijk linked the northern provinces of the country, Friesland and North Holland, separated until then by a natural estuary. Geographically, the dike faces the sea of ​​the Wadden Islands (Waddenzee, in Dutch), which reaches the coasts of Germany and Denmark and has been declared a World Heritage Site by Unesco. It is an environment of great natural wealth with sandy plains and strong currents, and the current works to reinforce the Afsluitdijk were the opportune moment to give a hand to the fish.

Under normal circumstances, the dock’s two large locks open for ship traffic and allow water to pass through. In both cases, it is not enough for there to be a uniform migration of marine species, and although the protection against the tides has been a success, “the barrier has been the biggest disaster in northern Europe in terms of biodiversity by losing Lake Ijssel most of its species due to lack of exchange with the sea, ”says Erik Bruins Slot, project manager and environmental engineer for the Province of Friesland. At low tide, the water is also thrown into the sea by other openings in the dike, although only a few fish have the strength to climb the current thus created in that period of time. The migratory river designed by this expert takes advantage of the movement of the tides in an exercise in ecological engineering driven by climate change, and estimates that some three million specimens will enter the lake annually.

Located in the Frisian part, the structure has an area of ​​50 hectares and consists of two parts: one on the sea side and the other on the inland lake; in the middle is a passage in the dike. In the first one, a stone basin with two mouths is prepared on the sandy ground. At low tide, the fresh water of the lake will cross the corridor of the dike to be expelled later by the mouths. Sensing it in the sea, the fish will be tempted to swim in that direction. The most resistant, such as sea trout and Atlantic salmon, will thus access the dock. The thorny and the chaplain, among other less robust specimens, will use the rising tide to enter because their mouths will remain open. Once inside, and still on the side of the Waddenzee, everyone will advance towards Lake Ijssel. In order not to compromise the safety of the dam in the event of a storm, a floodgate capable of withstanding even a storm surge is provided, which occurs when the wind raises the water above the usual sea level.

Once the dike is crossed, the fish will gain the other basin, already in the lake. There, a complex of about four kilometers awaits them that recreates a river course, with a dozen meanders made of sand – on the inside – and stone containments – on the outside. There will also be islets for birds to nest, as in an estuary. The turns that the fish must take will help them adapt to the environment to be able to follow the path of the Rhine, and other large flows that flow into the lake, to grow and breed. At the mouth of this designer river there will be another floodgate that opens and closes to the rhythm of the rise and fall of the sea. “As the flow of fish will be regulated naturally, it will not allow time for too much salt water to enter the lake,” adds the engineer. The deterioration of the meanders of the river and the periods of drought will also be monitored. The plan was originally encouraged by conservationist and fishermen organizations. Today, among others, the Ministry of Economy and the provinces of North Holland and Friesland participate.

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