About 80 percent of the approximately 28,000 fish species travel in schools. Fish travel in schools for both protection and for efficiency. Safety in numbers (in a school) is a form of predator avoidance, because trying to catch one fish in a large, moving school can be difficult for a predator. Secondly, fish that travel in schools have less drag (friction) and therefore use less energy for swimming. Also, when fish spawn, a school ensures that some eggs will evade predators and live to form another school.

The movements of a school of fish, which confuse predators, happen because the fish detect pressure changes in the water. The detection system, called the lateral line, is found along each side of the fish’s body. Along the line are clusters of tiny hairs inside cups filled with a jellylike substance. If a fish becomes alarmed and turns sharply, it causes a pressure wave in the water around it. This wave pressure deforms the “jelly” in the lateral line of nearby fish. The deformation moves the hairs that trigger nerves, and a signal is sent to the brain telling the fish to turn.