Fish that go back and forth through freshwater and salt water are called ANADROMOUS fish. Here are some of them. Alwives, sea lampreys,blue black herring, and smelt.
The reason some fish go back and forth through the different waters is to spawn. Spawning is when they lay there eggs. They lay there eggs. The eggs hatch. Then the babies travel to salt water. When they grow up come back and lay eggs and the cycle keeps on going.
here are some fish that go back and forth through salt and fresh water to lay eggs. Alwives,blueblack herring, sea lampreys, smelt, and most types of salmon and shad.