Bull sharks can survive in freshwater and frequent inlets and rivers throughout the world, including the Mississippi River, the Amazon River, Zambezi River, Nile River among many other bodies of water. Bull Sharks are able to adapt to freshwater due to their rectal gland, which can excrete salt from the body and controls their salinity level.
Notes from the University of Florida regarding Bull Sharks in Freshwater: “They have been reported 3700 km (2220 mi) up the Amazon River in Peru, and over 3000 km (1800 mi) up the Mississippi River, in Illinois. A population in Lake Nicaragua (Central America) was once thought to be landlocked, but it was subsequently determined that they gain access to the ocean through a system of rivers and estuaries. In the western Atlantic, bull sharks migrate north along the coast of the U.S. during summer, swimming as far north as Massachusetts, and then return to tropical climates when the coastal waters cool (Simpfendorfer and Burgess 2009).”
Nicarauga.com devoted a page to covering the bull sharks native to Lake Nicaragua, and it became the second most popular page on the entire site about the country. Below is an excerpt:
“Despite being a freshwater lake, Lake Nicaragua contains sharks that have adapted to freshwater life. This originally led scientists to believe that, due to the lake’s proximity to the Pacific Ocean, the area that the lake currently occupies was previously a giant bay. The theory for the presence of these sharks was that over a period of time, the gap closed forming a lake and trapping the sharks which then adapted as the water as it gradually changed from saline to fresh. Recent studies, however, have shown that the sharks have more than likely traveled up the San Juan River which connects the lake to the Caribbean Sea, jumping upstream in much the same way as salmon do.
Although scientist initially thought that the Lake Nicaragua shark was a separate species, it has been discovered that they are in fact bull sharks, so named because of their broad flat snout, stocky shape and notoriously unpredictable, aggressive behavior. This shark, which is called the Nicaragua shark in Nicaragua, is also known as the Zambezi or Zambi shark in Africa and Sundarbans or Ganges shark in India as well as a variety of names in other parts of the world.”