A Dutch "fish doorbell" has unexpectedly become an online sensation, blending slow TV with environmental activism.

In the central Dutch city of Utrecht, a "fish doorbell" has been installed at a river lock to assist migrating fish. Viewers of a live stream from an underwater camera can alert authorities when they spot fish as they journey to shallow spawning areas. By clicking a button, viewers send a screenshot to organizers, who notify a water worker to open the lock, allowing the fish to pass through.

Now in its fifth year, the initiative has attracted millions of viewers globally, offering a unique mix of slow TV and ecological engagement.

While the live feed often shows murky water with occasional bubbles, fish are more likely to appear as the water warms. Without intervention, native freshwater species like bream, pike, and bass may get stuck behind the lock and become vulnerable to predators, as the lock is rarely opened for boats during spring.

The concept was created by ecologist and concept developer Mark van Heukelum, who has been pleasantly surprised by the global response. "I think the combination of a good cause, a beautiful story, and a simple idea has sparked all this attention," he said.

Anna Nijs, an ecologist with the Utrecht municipality, was also taken aback by the worldwide popularity of the project. "We receive a lot of fan mail from people who find the slow TV calming," Nijs said, adding, "They also appreciate the chance to contribute to the cause."