“There are Bull sharks in Lake Nicaragua which have been there for generations. I don’t know of any Snapping Turtle or Muskie that can remove someone’s leg, it had to be a Bull.”
I don’t know this first-hand, but the evidence is that Lake Nicaragua is open to the sea. According to the Wikipedia entry for Bull Sharks there have been specimens tagged in the lake that were later found in the ocean and visa versa. Also, on the discussion of their evolved ability to tolerate freshwater there seems to be a definite limit when it comes to adult sharks. That they will eventually die, if only by starvation. But there may also be limits on toleration place on them via the functioning of key internal organs (ie. kidney & liver) that would kick-in long before starvation.
The theory is that Bull Sharks evolved to tolerate freshwater as a breeding-survival strategy. They go up into brackish water to have their pups, then move back into the estuary/sea. The pups grow up in freshwater, away from predators, and eventually move down to saltwater as they mature.
Interesting! I have read that the connection between Lake Nicaragua and the sea is a rather shallow waterway and during droughts, is impassable. I imagine that makes it impossible for larger sharks to leave or enter for long periods of time. I’ve also read that a shark fishery developed around the lake due to the sheer numbers of Bulls found within it. As a result, Nicaragua has banned catching the sharks.