Abstract
This study aimed to identify geographical distribution patterns of the giant short‐tailed river stingray Potamotrygon brachyura and the motoro stingray Potamotrygon motoro in the Uruguay River basin. Data on presence/absence of stingrays were based on fishers' knowledge accessed by interviews through expeditions in Brazil, Argentina and Uruguay. The significance in independent variables (environmental descriptors) of elevation ( p = 0.00 for both species), upstream distance ( p = 0.02 for P. brachyura and 0.03 for P. motoro ) and downstream distance ( p = 0.00 for both species) explained the presence of stingrays in the main water bodies of lower Uruguay and their absence throughout upper Uruguay more than the biogeographical barrier of the Salto de Yucumã ( p = 0.99 for both species) and basin area ( p = 0.42 for P. brachyura and p = 0.43 for P. motoro ) in the last steps. The construction of logistic models also provided high sensitivity (96.3%–97.5% for P. brachyura and 62.3%–71.7% for P. motoro ) and specificity (86.8% for P. brachyura and 85.4%–86.5% for P. motoro ) results between observed and presumed distribution for both species, with values of false positive and false negative varying between 1.3%–14.1% and 6.4%–9.2%, respectively. Further studies are still necessary not only in the Uruguay River basin or the La Plata basin complex, but for all South America, considering potential changes in the state of knowledge of freshwater biodiversity and its geographical distribution, including the possibility of undescribed species.