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Why do Fish Swim in Schools?

29 October 1997

 

Whether navigating the waters of freshwater lakes or migrating through the ocean, schooling fish draw benefits from their numbers. Schooling has been the subject of many scientific studies, resulting in a number of theories about this phenomenon.

Fish form schools for protection from predators; "safety in numbers" is especially beneficial for young fish and smaller species. Schools containing hundreds or thousands of nearly identical fish can confuse predators and make it difficult to single out and attack a particular one.

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Lookdowns are just one of the many fish species that form schools.  Schooling allows fishes to swim longer, travel through colder temperatures, and avoid predators
moreeffectively than if they swam alone.

 

Small fish in a dense school, moving in unison, may discourage a predator by appearing as a single, much larger creature.

 

Schooling makes it easier for fish to find food. With many more eyes to search for food, more meals will be found. By working as a team, a school may be able to seize larger food items than any one fish could manage to capture.

 

Another benefit of schooling is that it brings the sexes together and increases the odds of successful reproduction. Many fish species form schools only when it is time to mate.

Schooling also increases the efficiency of swimming for fish. Drafting in the wake of their schoolmates allows fish to conserve energy, swim longer and even consume less oxygen than they would if swimming alone.

 

Generally comprised of fish of the same age and size, schools typically face in one direction and exhibit synchronized movements. Each fish maintains an exact spacing from its neighbor. As they swim, they follow the movements of their neighbors and change their course in unison.

 

How do fish achieve harmony of movement in a school? Vision is the primary sense used to hold their place in a school. Visual markers play a big role -- each member of a school follows some key feature of the fish around it, usually a stripe or spot on their bodies, fins, or tails. Because of this dependence on vision, schools break up or at least lose their internal structure at night. The vibration-detecting lateral line, a row of sensory cells that runs along the sides of the body, also provides information about neighbors' movements.

Of the more than 20,000 species of fish, 16,000 are juvenile schoolers and 4,000 school as adults. Schooling species familiar to North Carolina's freshwater include threadfin and gizzard shad, golden shiners, white bass, and white bass-striped bass hybrids. North Carolina marine schoolers include herring, mackerel, bluefish, mullet, jack, pompano and menhaden.