Density stratification based on salinity (or temperature) is quite common. In fact, it was well demonstrated on the TV show Planet Earth.I (not qualified) think however that the bodies of fresh and salt water would mix? And as I stated earlier, what would keep the salt water fish from rising into the fresh water layer? My understanding is that fish have swim bladders that keep them at the right depth.
What would keep the fish from rising into the fresh water layers is that they would be uncomfortable in fresh water, so they would swim back to a level at which they were comfortable. It is my understanding that swim bladders can be emptied and filled, and is actively controlled by the fish.
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