Utrecht's "fish doorbell," an online live stream at the Weerdsluis lock, has become a global hit, blending slow TV with ecological activism. The underwater camera lets viewers spot fish and alert authorities to open the lock, aiding their spring migration.

Now in its fifth year, the project, created by ecologist Mark van Heukelum, has drawn millions of viewers. Many find it relaxing, while others appreciate its role in protecting fish like bream and pike from predators. Utrecht ecologist Anna Nijs is amazed by its global appeal, calling it a simple yet impactful initiative.