Translated from Dutch NOS TV today:
Human interventions have made it more difficult for fish to swim to the sea through rivers, canals and lakes. Sluices, weirs and dams are obstacles to the movement of fish between fresh and salt water and vice versa.
To keep the migration going, fish passages have been built near obstacles, but these lead to new problems. The passages are so small that fish have to wait for each other causing traffic jams. Especially small fish like juvenile eels and three-spined sticklebacks that way are easy prey for predatory fish and birds.
In and around the North Sea Canal something is done about that. In the vicinity of the passages concrete pots with holes are placed, in which small fish may hide. Today, the first pots are placed in the Noorder IJplas lake.