The Weerdsluis boat lock is rarely opened in spring (Credit: CC-BY-SA-2.0)

Every spring, many fish species migrate from the Vecht, a branch of the Rhine River, to the shallow waters of the Kromme Rijn River to spawn and reproduce. However, their journey often stops at the Weerdsluis lock in Utrecht, Netherlands. The manually operated lock, designed to maintain the canal's water levels, rarely opens in the spring months. But now, thanks to an interactive "fish doorbell," the marine animals are able to reach their destination with little or no disruption.

Dutch ecologist Mark van Heukelum came up with the ingenious idea in 2021 after a conversation with the lock manager.

"I was sitting there with the Weerdsluis boat lock manager, and I was telling him about the challenges fish face when they are migrating. I mentioned that this boat lock was actually an obstacle for them," van Heukelum says. "We could literally see the fish swimming there at the boat lock. I told him that this boat lock should be opened during the spring for the fish."

The Weerdsluis boat lock has to be manually opened to allow fish to pass through (Credit: Mark van Heukelum/ CC-BY-SA-2.0)

The manager told van Heukelum that he would open the lock if he could see the fish. To enable that, the ecologist placed an underwater webcam near the lock. He then set up a website for the live stream and asked the public to help him by ringing a digital "doorbell" to alert the lock manager if they saw a fish.

To his delight, 100,000 people from across the world rang the "doorbell" during the four spring months in 2021. Things have only improved since. In 2023, over 8.2 million people visited the website, and the bell was rung over 105,000 times! The 2024 season began on March 1 and is already off to a great start. In addition to notifying the lock keeper, excited visitors also compare notes and share photos of their fish sightings on social media sites like Facebook.

Many fish species have been seen in the live feed (Credit:

The bell can be replaced with an automatic sensor to detect the fish if people ever lose interest in the project. But until that happens, van Heukelum wants to keep the public involved in this worthwhile cause.

"I do appreciate the value of automation, but I just think it's beautiful when the public comes together to do something good," says van Heukelum.