Freshwater Sharks
Talking about freshwater sharks may sound like a contradiction in terms. After all, everyone knows that sharks live in the ocean. Even the tiny freshwater sharks you see in aquariums are really in the minnow family, not the shark family. But, freshwater sharks do exist. And, they exist in a couple of different ways. First of all, there are sharks that leave the ocean and live for years in rivers and lakes. Some people call these freshwater sharks. Then there is also a whole family of sharks called river sharks. These are truly freshwater sharks because they never travel to the ocean.
The Bull Shark is considered by some to be both a freshwater shark and a saltwater shark. That’s because Bull Sharks travel in and out of the ocean. They can live in freshwater for many years. The thing is, however, a Bull Shark cannot reproduce without being in salt water, so it visits the ocean to breed and then, many times, makes a return trip back to rivers and lakes.
The young have to be born in the ocean, but then the mother shark may go back to fresh water to raise her young. They are much easier to protect in a lake, for example, because lakes do not have the number of predators they would see in the ocean--particularly the sharks that eat other sharks. A Bull Shark can live in a lake or river not because it is created differently than other sharks, but because its body has an instinctive mechanism enabling it to adapt to fresh water. They can adapt to different levels of salt by being able to change the salt content of their own bodies. They lower the amount of salt and urea (protein waste) in their bodies by peeing more. When in a lake, a Bull Shark pees twenty times more often than it does in the ocean.
Bull Sharks have been seen in many lakes and rivers. In 1955, a shark was reported as being seen in Lake Michigan. Other sharks have reportedly been seen in the Mississippi River. In the aftermath of Hurricane Katrina, large groups of Bull Sharks were seen in Lake Pontchartrain. In a study completed by University of Nebraska researchers, Bull Sharks were studied in Lake Nicaragua, the biggest lake in Central America. They found that not only did Bull Sharks live in the lake but they migrated yearly to the ocean and then back to the lake. They found this out by tagging and tracing the movements of sharks. The sharks not only returned to the lake, but like salmon, jumped eight different sets of rapids to get there. Many of these sharks traveled 118 miles in a week.
While some scientists disagree on whether or not to call Bull Sharks freshwater sharks, there is no controversy about river sharks. There are a total of six kinds of river sharks in the Glyphis family. They are rare and so far only three out of the six have been given names. They are often confused with the Bull Shark. River sharks live in Australia and Asia. The Ganges Shark makes its home in India and Pakistan. The Speartooth Shark lives in Queensland, Australia and in New Guinea. The third named shark, the Irrawaddy River Shark, can be found in Rangoon, which is in Myanmar, formerly called Burma. The yet-to-be-named sharks are in Australia and Borneo.
River sharks are distinguished from ocean sharks because they have different fin sizes and shapes. Also, their lower teeth stick out so that they can be seen when the shark’s mouth is closed.