Fish can “sense” the movement of other fish in their school by using something called the “lateral line”
Fish can form schools containing thousands or millions of fish. They are able to stay together by using their eyes and something called a “lateral line”; this enables them to hold their places in the school. The lateral line is a row of pores running along the fish’s sides from their heads to their tales. Special hairs in these pores are able to sense changes in water pressure from the movements of other fish in their school or even predators.
The lateral lines are usually visible as faint lines running down the length of the fish on both sides, commencing at the gill covers to the base of the tail.
In some species the receptive organs of the lateral line have evolved to function as electroreceptors, these are organs that are used to detect electrical impulses, and as such these two systems remain closely linked.