On a damp and grimly grey winter day in Paris, Bill François is a beacon of positivity. After a brief stroll along the River Seine, we come to a halt at the Henri IV Dock with Notre Dame cathedral just about visible through the mist. Here, François flicks his wrist back and then gently launches a fishing line out into the choppy waters. “The Seine is a wild place in the heart of Paris,” enthuses François, who has come prepared in an all-weather outfit that includes a cap, sunglasses and waterproof pants. “It’s like an aquarium. The water is quite clear these days and the fish are multiplying.” François, a marine biologist by training, fishes in the Seine several times a week in different locations, surveying what he catches for long-term population monitoring by Parisian fishing associations. And year by year, François and other fishers are finding more and more.  Bill François with a pike he caught in Paris. Courtesy of Bill François In fact, in the 1970s there were only three fish species left in the 777-kilometer (482-mile) river, which spans northern France and cuts through the capital city. But after decades of urban water policies alongside community efforts, there are now nearly 40 that have been officially registered, with new species turning up all the time. Beyond fish, according to François, there are also other kinds of creatures such as jellyfish, crustaceans, sponges, shrimp and kingfishers coming back to the Seine. “The restoration efforts of this river have been a real success,” adds François. The Seine’s thriving biodiversity is largely due to an improvement in urban wastewater treatment and the quality of water that is now being discharged into the river, according to French officials. “We were pioneering almost by obligation,” says Vincent Rocher, who co-authored a report published in November 2024 for SIAAP, the French public utility that has been tasked with collecting and treating wastewater in the Paris region since 1970. Thousands of years ago and even up to the Middle Ages, Paris was a city of marine life, the Seine — which is over 14,000 years old — teeming with eels and salmon. But the municipal sanitation network became vastly inadequate as the city industrialized in the 19th century and as its population rose from about four million in 1900 to 12 million in 2020.  “We dumped a lot of organic matter in the water over that period,” explains Rocher. “The population growth far exceeded our clean up capacity. So the Seine was biologically dead, there wasn’t oxygen for wildlife. The fish almost disappeared.” Such was the dire state of Parisian waterways that one little-known fluvial cousin to the Seine, the Bièvre, was gradually concreted over and eventually buried by 1912. (Efforts are currently underway to restore the river.)