According to North Carolina Aquariums, fish swim in schools because schooling protects them from predators, encourages reproduction and makes it easier to find food. Schooling also conserves energy, as each fish drafts in the wake of the fish ahead of him. This makes it easier for fish to swim long distances without exhaustion.
North Carolina Aquariums reveals that there are 20,000 species of schooling fish in marine and freshwater environments. Some fish always travel in schools, while other species restrict schooling to juveniles or to mating season.
Fish schools contain hundreds of individuals moving in the same direction at the same time. The fish rely heavily on their eyesight to stay in their proper place and move with the rest of the school. Because vision plays such a crucial role in schooling behavior, most school movements occur during daylight hours.
Schooling fish also rely on their lateral lines to detect vibrations from the fish around them. This ability keeps each fish at a precise distance from adjacent individuals and preserves the internal organization of the school. Rhode Island Sea Grant explains that this prevents fish from crashing into one another and bolsters their defenses. In the eyes of potential predators, tightly packed schools appear to be a single huge creature. Even predators with excellent vision have difficulty attacking fish schools because they cannot zero in on a single target.