A bull shark can enter freshwater, but TuffShark found no credible scientific record of one in the Great Lakes. The route is long, engineered and obstructed, while the lakes are far colder than the warm habitats bull sharks tend to use. Possible on a map is not the same as documented or likely.
Freshwater is not the problem
Bull sharks are euryhaline: they can regulate salts and fluids across a wide range of salinities. They regularly use estuaries and rivers, and young sharks can spend substantial time in low-salinity nursery habitat.
That remarkable physiology explains why a Great Lakes rumor sounds plausible. It does not, by itself, supply evidence that the trip has happened.
There is a connection—but not an open highway
The Chicago Sanitary and Ship Canal created an aquatic connection between the Mississippi River basin and Lake Michigan. A shark moving north would still face an enormous river journey, locks and altered channels.
The U.S. Army Corps of Engineers also operates electric dispersal barriers in the canal system to reduce the risk of fish moving between the two basins. A line on a map leaves out the engineering.
Temperature changes the odds
Freshwater studies show bull-shark habitat use depends on more than salinity. In one analysis, juveniles showed the strongest affinity for warm water around 29–32°C alongside other favorable conditions. The Great Lakes are seasonally much colder.
A brief presence and a self-sustaining population are also different claims. Even if an individual somehow completed the route, cold winters, habitat conditions and the absence of a breeding population would remain major obstacles.
CHECK THE EVIDENCE
Primary and authoritative sources
A peer-reviewed analysis of the environmental conditions associated with juvenile bull-shark habitat use.
Official description of the aquatic connection between the Mississippi basin and Lake Michigan, including operating fish barriers.
Species profile covering freshwater tolerance, range, size and life history.