Eco aquaduct in North Holland.

Around 1927  the  ‘Afsluitdijk’’, an enormous  32 km long dike was built in  Holland to connect the northern provinces of North Holland and Friesland. The dike was meant to protect the lowlands in the interior of Holland against the floods during the violent Western and  North Western storms. With its construction the former salty Zuiderzee was transformed into two large sweet water lakes, the IJsselmeer and Markermeer. The new dike not only ended commercial fishery and their ports, but also created  a blockade of the migratory routes of millions of fish  like stickelback, bream and eel. In particular of  the baby eels (‘elvers’) on their yearly 4000 km route from their place of birth in the Sargossa sea near Bermuda, to Holland 

The Dutch water management  recently created a passage in the dike to restore the natural migration of fish between salt and sweet water.  The new fish passage is located on the Westside of the locker (‘sash lock’) in Den Oever (see insert). Its construction is simple but effective.  In the container at the lower left  sweet (light bue) water from the Ysselmeer is pumped above the level of the brackish (darker blue) water of the Waddenzee. Thus creating a constant stream of sweet water flowing through a tunnel to the Waddenzee. The fish that are already crowding in the Waddenzee  are then tempted to swim against the current  through the tunnel to to the IJsselmeer.  The success is enormous:  not only baby eel but thousands of smelt, bream  and sticklebacks  now daily use the passage. In some nights 30.000-50.000  baby eels have been observed passing by. One estimates that per year hundreds of billions of fish will start using the new passage. The plan for the future is  even more promising, namely to build an aquaduct at the other side of the dike in Friesland, allowing fish to move freely with the tides between the salt and  sweet water.