This is a line of special neuromast cells that runs down either side of a fish body. These two lateral lines are highly sensitive to movements and the displacement of water as the fish swims close to its neighbor. They aid in keeping the fish in a neat, orderly pattern. Some fish do not have lateral lines, nor the sensitive cells, and thus rely on their eyesight. The lateral lines are especially important to fish living in the highly murky waters of the estuarine environment where sight is not particularly useful.
Fish stay with their own kind in a schooling configuration.
Schooling has some other interesting aspects. They begin just where the rivers pour into estuaries and then, seemingly without a central signal, they migrate up the rushing currents. So large are their numbers that the bottom of the stream cannot be seen, and the whole picture is one of wriggling, bluish bodies swimming against the current. In many ways fish schools are much like herds of land animals or flocks of airborne birds. There is that undefined need to stay together.