Movement of the bull shark (Carcharhinus leucas) in the upper Mississippi River Basin, North America

Rare appearances of bull sharks (Carcharhinus leucas) in river systems and other freshwater ecosystems have been reported from five continents. The wide geographic range of this phenomenon, the physiological adaptations of this species to allow for movement into rivers, and a fossil record in the Miocene Epoch, all imply that this behavior has a long history. In the Mississippi River basin, however, only two specimens were captured in the river’s upper portion during the entire 20th century. Further historical, archeological, and paleontological records of these animals appearing farther than 500 km upstream from the Mississippi delta is almost nonexistent. Bull shark movements in the upper portions of this river are likely sufficiently rare or cryptic to avoid detection using historical, archaeological, paleontological, and modern fisheries conservation techniques. Deeper research into these sharks, and their historical biogeography is needed to better understand the relationship these large predators have to the freshwater ecosystems of the American mid-continent.