A 74-centimeter juvenile female stingray, tagged in a quiet backwater of Argentina’s Paraná River in April 2025, reappeared ten months later nearly 170 kilometers downstream—an extraordinary journey that has rewritten what scientists thought possible for freshwater rays. This single fish, a giant freshwater stingray (Potamotrygon brachyura), traveled farther than any obligate freshwater ray ever recorded, shattering the long-held belief that these animals are largely sedentary. For decades, conservation strategies assumed that species confined entirely to freshwater—unlike their salt-tolerant cousins—moved only short distances, rarely exceeding a few kilometers. But this discovery, made possible through a collaborative tagging program involving scientists and recreational fishers, reveals that some freshwater rays are capable of movements once thought exclusive to ocean-going sharks and rays like bull sharks and sawfish.
The implications are profound. Freshwater elasmobranchs, including stingrays and river sharks, are among the most endangered vertebrates on Earth, yet their behavior remains poorly understood. The giant freshwater stingray, categorized as Vulnerable by the IUCN Red List, inhabits fragmented river systems increasingly obstructed by dams, pollution, and overfishing. Until now, conservation efforts focused on local scales, assuming these animals didn’t need large, connected habitats. But this stingray’s 170-kilometer trek—averaging 579 meters per day—suggests otherwise. It traveled through a complex network of channels and mainstem rivers, navigating a dynamic and increasingly altered landscape.
What makes this journey even more remarkable is the species itself. The giant freshwater stingray is the largest obligate freshwater ray in the world, capable of reaching 1.8 meters in width and weighing over 200 kilograms. While smaller relatives like Potamotrygon motoro and P. falkneri rarely move more than 8 kilometers, this individual’s size may have enabled greater mobility, aligning with patterns seen in other large freshwater fish. The finding raises urgent questions: Are such long movements rare anomalies, or do they reflect a hidden capacity for dispersal across populations? How might dams and river fragmentation disrupt these movements, cutting off access to spawning or feeding grounds?
Led by researchers Diego M. Vazquez and Luis O. Lucifora, the tagging program in the middle and lower Paraná River continues to gather data with the help of local anglers. Each recapture adds a piece to the puzzle of how these elusive animals live and move. As river systems face growing pressures, understanding the true scale of their movements is no longer just a scientific curiosity—it’s a conservation imperative. If even one juvenile female can travel 170 kilometers, then protecting her species means protecting entire river corridors.