Sincerely, Marcelo Paes, Brazil. There are many families of fishes that have or part of their life cicle at one of the sides or others that lives in the estuary. Fishes that run away from the continent are called anadromous, and that run after, catadromous. That fishes that lives there are called diadromous. They can balance the effects of salinity by osmorregulation.
Bony fishes have chloride cells, that filter the salts balance.
So, some species of sharks, rays, herrings, anchovy, catfish and others can be at both sides.