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.
I’ve never been to Nicaragua, but I have been to Honduras and Costa Rica. There are a “rainy” and “dry” seasons down there, so I’m thinking that the rivers do tend to run low. So maybe it’s like the salmon run when the female bulls wait for the rains to make the rivers passable?