Why fish shoal

It looks so perfect! How is this achievement possible? First, it is important to understand the difference between a shoal and school of fish. Sometimes people don't really know what is the distinction.

This two terms are routinely used in marine life books and in bio-conversation media, and both of them are used to define a behaviour of a group of fish. A group of fish staying together is called shoal when its purpose is to avoid being eaten by a bigger predator.

A shoal is a group of individuals congregating together to benefit from "safety in numbers" but not moving or behaving in unison. The reason they stay in a group can be as well for social reasons: meet a female or a male partner, or to find food. They prefer to find a healthy shoal of fish, with mates from the same specie, or same colour and size. If the shoal acts in unison, then it becomes a school of fish.

Acting in unison means: Changing direction at the same time, staying close to each other and heading on the same direction.

A school is an organised shoal of fish. Big schools make often the predators confused, because those cannot be focused on one prey only. Even sometimes, the predators themselves become afraid. Schools of fish move very fast and change direction in an instant neither breaking the formation nor hitting each other.

The speed of reaction allows them to copy their neighbours at the same time, which they can see from both sides. The vision is a very important requirement for the school to be able to survive. However, many species of fish have a lateral line on their body that contains similar cells from the ciliated ones of our internal ear.

These specific cells allow the fish to detect changes in the ocean currents and in the movements of close individuals when the visibility underwater is low. In a shoal of one hundred fish, each individual has only a one-in-one-hundred chance of being killed during an attack.

This benefit, as with foraging success and mate acquisition, increases in value as the size of the shoal increases and maybe another reason why fish choose large shoals over small shoals. Another important consideration, with respect to the anti-predator benefits of shoaling, is the confusion effect. This phenomenon is associated with the high degree of similarity between members of a shoal.

In theory, the confusion effect suggests that a predator, confronted with a large group of similar-looking individuals, experiences perceptual confusion, which slows the attack. This may occur because the predator has difficulty identifying a single individual fish to attack. For example, imagine a bright yellow fish shoaling with a group of grey fish.

An approaching predator will be drawn to the yellow fish as it stands out in stark contrast to the rest of the group. Since fish change shoals on a regular basis, individuals are often faced with the decision of which shoal to join. Joining the wrong shoal could have disastrous consequences. Most shoaling studies, however, indicate that fish are very good at choosing shoalmates with similar characteristics to themselves.

The study of shoaling behavior is relatively easy. Test tanks like the one used in Fish Cam are aquariums divided into thirds by the addition of two panes of glass. During an experiment, the time the test fish spends near each end chamber is recorded. Increased time near one end indicates a preference for the shoal in that chamber. When you open Fish Cam you will see a standard gallon aquarium that has been divided into thirds with the addition of two glass panels, eight inches from each end wall.

The central chamber is partially divided by two opaque baffles. The test fish can move freely through the central chamber, but cannot see both end chambers simultaneously. Each day we will add a new test fish to the central chamber.