In the cold, dark waters of some French rivers lives a fish that almost no one has ever seen in their lifetime. Known as the Rhône loach, it is a nocturnal and elusive creature that has become nearly invisible to the public. Once widespread across France, it is now critically endangered, with its population reduced to just a fraction of its former range. The Rhône loach, whose scientific name is Zingel asper, is found only in the Rhône basin and nowhere else on Earth. This small freshwater fish, rarely exceeding 20 centimeters in length, has an average lifespan of three to four years, spending most of its time hidden beneath pebbly riverbeds during the day. The Rhône loach was once present in about 2,200 kilometers of watercourses across the Rhône basin at the beginning of the 20th century. However, by 2010–2012, its range had shrunk to just 240 kilometers, a loss of 90%. This dramatic decline has left the species with only a few remaining habitats, and it has disappeared from nine out of ten rivers where it once lived. The reasons for this loss are manifold, including the construction of dams, pollution, and the warming of river waters. These factors have disrupted the fish’s natural habitat, fragmenting populations and reducing genetic diversity, which makes the species more vulnerable to environmental changes. Despite the severity of its decline, the Rhône loach has not been hunted or fished extensively. Instead, it has gradually lost the conditions necessary for its survival. Dams and artificial canals have fragmented the rivers, isolating populations and limiting their ability to reproduce. Water pollution, particularly from agricultural runoff and chemical residues, has further degraded the quality of its habitat. The warming of the Rhône basin’s waters has also created unfavorable conditions for the fish, which is adapted to fast-flowing, well-oxygenated environments. Efforts to save the Rhône loach have included two National Action Plans introduced in 2011 and 2012, aimed at halting its decline and restoring its habitat. These plans have brought together around thirty measures, including the modification of dams to improve ecological continuity and the reintroduction of nearly 22,000 fry into the Drôme river. Today, the Rhône loach survives in only a few isolated sections of rivers such as the Loue, the Ardèche, the Drôme, and the Verdon. The future of this unique species remains uncertain, and whether these conservation efforts will be enough to prevent its final disappearance is still to be seen.