There are persistent rumors of sharks in the Mississippi River and in the Great Lakes. There’s a problem with those reports. With one known exception, sharks cannot survive very long in fresh water, and the temperatures of the waters in winter would quickly kill any shark present then.
The one exception is the bull shark, Carcharhinus leucas. There is at least one confirmed incident of a bull shark in Illinois. In 1937, two fishermen near Alton, Illinois, caught a five foot bull shark. There is photographic evidence of the incident.
Bull sharks have special physiological adaptations allowing them to survive in freshwater. In 1972, a bull shark was caught 2,500 miles up the Amazon, in Peru. It is thought that this ability to survive in freshwater environments allows small bull sharks to evade predators by moving into freshwater. Nothing prevents them from taking up residence in tropical freshwater lakes, as they have in Lake Nicaragua in Central America, but the cool to cold seasonal temperatures in most American lakes would prevent it.
Sharks in Lake Michigan terrorizing Chicago beaches is a popular meme, but is entirely untrue. The only known sharks in the Great Lakes are in a Chicago aquarium. The bull shark caught in 1937 remains the only authenticated incident of sharks in the Midwest.
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