Since 1984, France has introduced fish passes on its rivers to help migratory fish like salmon, shad, and eels swim upstream to their spawning grounds. These structures, such as fish ladders or bypass channels, have been widely implemented in response to the 1984 inland fishing law. Today, nearly 60,000 dams, locks, weirs, and old mills block rivers across the country. EDF Hydro, which operates over 400 hydroelectric power plants and 600 dams, has installed more than 250 fish passage devices, especially for migratory species. On key river sections, 150 dams and power plants have been equipped with over 190 fish passage structures designed for both upstream and downstream migration. Despite these efforts, the downstream migration of young fish, known as smolts, has remained a significant challenge. While engineers have developed "ichthyocompatible" devices—such as fine mesh grids combined with surface outlets—to guide fish away from turbines, their effectiveness is limited. A test on the Gave d'Oloron river showed that even with a grid featuring 20 mm spacing and two surface outlets, only a small portion of the fish were diverted from the turbines. The majority of the water flow continued directly to the turbines, leaving the backup outlets unable to attract most of the fish. The impact of multiple dams on fish survival has been studied in the Loire-Brittany basin. Models indicate that salmon smolt mortality is estimated at 27% in the Loire, compared to just 2% in Brittany. This stark difference is attributed to the higher number of dams and hydroelectric developments in the Loire, which increase cumulative fish mortality. While fish ladders help fish navigate upstream, they can become dangerous obstacles during the downstream journey, where young fish are at risk of being swept into turbines. Efforts to improve downstream passage have been ongoing. As early as 2015, the former Onema (now part of the French Office for Biodiversity) began evaluating the effectiveness of downstream passage devices for Atlantic salmon smolts, aiming for passage efficiencies exceeding 90%. However, these specialized and costly structures are only implemented on a small fraction of the thousands of dams and weirs across the country. Their widespread use faces similar challenges to those encountered with upstream fish passes four decades ago. Experts emphasize that true progress requires a basin-wide approach, as each additional dam contributes to the cumulative mortality of fish. Until downstream migration is considered a core design factor in river infrastructure, fish ladders will continue to aid upstream movement without ensuring safe return journeys for migratory species.