The fish doorbell was created to help fish that want to go to the Kromme Rijn to lay eggs. Particularly in early spring, when there are still few boats, the lock sometimes remains closed for a long time and the fish cannot go any further.
The fish doorbell is not a real doorbell, but a website on which you can see live underwater images of the lock. Do you see a lot of fish? Then you can press a button and the lockkeeper will receive a signal to open the lock.