A group of fish moving together can look almost like one giant animal.
The fish turn at nearly the same time.
They match speed.
They maintain spacing.
That coordinated movement is called schooling.
Schools can confuse predators
A predator trying to attack one fish has a much harder job when hundreds of similar bodies are moving together.
The large group can make it difficult to focus on one target.
That lowers the chance that any particular fish will be caught.
More eyes can detect danger
A fish in a group does not have to notice every threat by itself.
Other fish may detect a predator first.
Their movement can trigger an immediate response through the group.
That creates an early-warning advantage.
Schools can help find food
A large group can search more area and respond when members locate food.
Fish may also benefit from information created by the movements of nearby individuals.
The group becomes a network of sensors.
Does schooling save energy?
In some species and formations, swimming near other fish can reduce the energy required to move through water.
Fish can interact with the water currents created by neighbors.
The exact effect depends on spacing, species, speed, and formation.
How do fish coordinate so quickly?
Vision is important.
Fish also use a sensory system called the lateral line.
The lateral line detects movement and pressure changes in surrounding water.
That helps a fish know where nearby animals are moving even when visibility is poor.
Is every group of fish a school?
No.
A shoal is simply a group of fish staying together socially.
A school is more coordinated.
The fish align, match speed, and move together in an organized way.
So they are not following one tiny fish who was elected leader that morning.
The whole group is constantly reacting to its neighbors.
A related animal question is How can fish breathe underwater?, which shows another way biology shapes familiar behavior or anatomy.
And that’s the short explanation.
Related explanations: How can fish breathe underwater?, Why do birds fly in a V formation?, and Why do penguins huddle together?