One of the most curious and mesmerizing things to observe in the sea are schools of fish: groups formed by hundreds, or thousands of individuals moving together and coordinated, swimming at the same distance and shining in unison!

Many marine species form schools, and the best known are mullet, herring, anchovies, sardines, tunas, and barracudas.

However, a school of fish is more than just a multitude of fish: it is a social organization to which individuals are bound by stereotyped behavior and even anatomical specialization. In such aggregations, fish are usually of the same size and age: in the same species, usually, the speed at which individuals swim increases with the size, and therefore they tend to sort themselves out and aggregate with their peers. Schooling fishes do not merely live close to their kind, as many other fishes do, they maintain, during most of their activities, a remarkably constant geometric orientation to their fellows, heading in the same direction, their bodies parallel and with virtually equal spacing from fish to fish.

Swimming together, approaching, turning and fleeing together, all doing the same thing at the same time, they create the illusion of a huge single animal moving. This peculiar social organization has no leaders. The fish traveling at the head of the school frequently interchange with those behinds them. When the school turns sharply to the right or left, the fish swimming on the sidelines become the “leaders” and vice versa.

Have you ever wondered how this is possible?

No, fish are not telepathic, and no, there is no one controlling them by radio!

Instinct plays a fundamental role in the formation of a school and, somehow, individuals communicate with each other and exchange the visual, acoustic, and chemical information that is needed to move in unison. There are two simple rules for all individuals in a school:

1. Stay close, but not too close to neighbors.
2. Keep swimming

In the event of a change of direction, fish see their neighbors and react to continue to hold their place in the aggregation. In addition, fish have a sensory organ called the lateral line that runs along each side of the body, from the structure that covers and protects the gills – the operculum – to the base of the tail (figure 2). This organ can detect very quickly imperceptible vibrations and pressure changes in the water when other fish accelerate or change direction. Fish at the head of the school tend to react first to external stimuli, and then the rest take their cues and follow suit, simultaneously adapting to the movement of the aggregation.

Still, the collective behavior of schooling fish needs to be better investigated to understand how it is possible to move in such a synchronized manner.

Why do they do it?

No, it is not just because they like to enjoy each other’s company!

School is a grouping of individuals that move together for strategic and survival purposes.
With all these eyes, schooling fish have an easier time finding food than solo swimmers. Swimming in a school is less exhausting, and fish can save a lot of energy if they can draft off their neighbors, taking advantage of each other’s momentum and achieving greater hydrodynamic efficiency. Being part of a school makes it easier to find a mate, thus increasing the chances of reproduction.

Another key function is a defense against hungry predators: by grouping, the fish become a “single individual” of larger proportions than the common predators, which increases the possibility of evading them and ensures the group’s survival. To elude predators, they react with super-quick movements, creating small groups that stand out from the school, reorganizing their shape, or even end up in a gradually widening circle.

And you, have you ever been mesmerized by watching a school of fish?