Great-now im going to be scared about swimming in lakes!

FRESHWATER SHARKS

Someone once told me about these freshwater sharks in Lake Nicaragua and I always meant to look it up, but forgot of course. However Lake Nicaragua isn't the only place that has freshwater sharks, although it is the most common and the largest. The most common freshwater shark is a bull shark, and on rare occassionthey have even been spotted in the Mississippi River. It certainly makes you think twice about swimming in freshwater lakes.

Bulls make their way into freshwaters to keep their newborn pups safe from harm. They don't mate or actually give birth in freshwater -- they do this in nearby tidal creeks that have a mix of salinity. The freshwater lakes and rivers provide safety for their pups, which would be hunted by larger predators in the ocean. Bulls have a physiological feature that separates them from other sharks and allows them to process freshwater and saltwater by manipulating the amount of sodium and urea that their bodies need. Urea is the waste produced by the body when proteins are metabolized. When bull sharks reduce the amount of salt and urea in their bodies, they are able to adapt to various levels of salinity in water bodies. How do they accomplish this? By peeing a lot -- roughly 20 times as much as when they're in the ocean. When bulls swim into freshwaters, they need to achieve a balance of saltwater to freshwater that still allows them to function normally. They're able to reduce the concentration of salt and urea in their blood by increasing their urine production, which is expelled in a much more diluted concentration than it normally would be. However, this overworks the kidneys of the bull shark, forcing it to travel back and forth between the ocean and inland waters.
http://animals.howstuffworks.com/fish/bull-shark2.htm

heres a you tube link for a video on bull sharks - http://www.youtube.com/watch?v=E9kkfX1d6N4