The construction of the Afsluitdijk in 1932 was highly beneficial to the Netherlands in terms of safety and economic development, but it did have a negative impact on fish. The two major nature conservation areas in the Netherlands – the Wadden Sea and the Zuiderzee – suddenly became separated. Routes for migratory fish to the European hinterland were blocked, and fish stocks dwindled. Rijkswaterstaat [Directorate-General for Public Works and Water Management] is therefore taking measures to allow both freshwater and saltwater fish to swim through.
Fish-friendly discharge sluice management
The Afsluitdijk contains 25 weepholes. These are large holes in the dam with a sliding partition in front. These holes are what Rijkswaterstaat uses to discharge the surplus water from the IJsselmeer into the Wadden Sea twice a day. Millions of litres of water then flow through these discharge sluices within a short space of time. On these occasions, fish can swim downstream through the lock into the Wadden Sea, but the current is too strong for most fish species to swim against the current to the IJsselmeer. They cannot migrate upstream to the IJsselmeer to reach their spawning grounds or juvenile habitat, and for this reason, Rijkswaterstaat has also opened several discharge sluices, as the height difference is relatively small, making the current less strong. This allows even the weaker swimmers to get back and forth between the IJsselmeer and the Wadden Sea.
In 2014, Rijkswaterstaat carried out research into how the Afsluitdijk can be reopened for fish using fish-friendly sluice and lock management. The research was focused on the discharge sluices and navigation locks. One of the objectives was to research how even the weaker swimmers could swim back and forth between the IJsselmeer and the Wadden Sea. The results are very promising.
Further measures have been taken in addition to fish-friendly sluice and lock management; Rijkswaterstaat is constructing a fish passage at Den Oever, and the Nieuwe Afsluitdijk project is working on a plan for a Fish Migration River at Kornwerderzand.