This is a fish doorbell. There is a watergate in Europe somewhere that leads upstream a river. The fish mate upstream, but to do that they must pass the gate. And since the gate is manual, they must know when the fish are there. So there is the fish doorbell.
If you see a fish you just press the button below it. The gate opens for around 30 minutes twice a day once enough fish have gathered. It’s fun to watch the fish swim around.
