Bull sharks have a unique ability to live in either salt or fresh water, and they are seen on occasion in Lake Michigan. According to Shark Savers, they accomplish this by using a process called osmoregulation. This process keeps the water level in the animal constant, and it reduces the influence of the environment. They reach Lake Michigan by navigating up the Mississippi River, which connects with the Illinois River.
Bull sharks are one of only two species of shark capable of living in fresh water. This ability allows the shark to alter its level of salt to adjust to freshwater. When a bull shark enters fresh water, it urinates much more to reduce the urea and sodium in the kidneys. This enables the shark to survive the change in salinity. Bull sharks enter fresh water to keep their newborn pups safe from predators. Lakes also offer good hunting for sharks and their pups.
Bull sharks do not give birth in fresh water. To give birth or mate, bull sharks prefer tidal creeks, which are a mixture of salt and fresh water. In waters around the United States, they live in the Gulf of Mexico and along the east coast, according to the National Wildlife Federation.
How Stuff Works explains that, following Hurricane Katrina, many bull sharks entered Lake Ponchartrain in Louisiana. Bull sharks commonly travel 118 miles to Lake Nicaragua via the San Juan River and far up the Amazon River. They also live in South Africa in St. Lucia Estuary, which is considered one of the saltiest bodies of water in the world.