Almost 80% of adults of the known species of fish swim in schools, at least during part of their life cycle, and they do this mainly because of two simple facts of nature: 1. Big fish eat little fish, and 2. There is safety in numbers.
So if little fish want to survive and reproduce in large numbers, their best option is to band together and swim in unison almost as if they were one single, and very big, fish. The side of the shape, and often the flashing of light reflected off thousands of fish can confuse predators, but being in a school also means that predators cannot generally eat the lot, and so significant numbers survive.
The downside for schooling fish is the human fishing industry, which exploits the schooling behaviour in fish species such as cod, mackerel and tuna to catch large numbers of fish at a time.
There are other reasons for fish to swim in schools. One reason is that as they swim and their tails sweep from side to side, they set up tiny whirlpools or vortices in the water which effectively reduce the friction for each of a fish’s neighbours. Less friction means less effort and energy is required to swim.
Another reason for fish to swim in schools is to act, not as one large prey species, but as one big predator. Bluefish are an example of this, since this species travels in schools in search of their prey the menhaden, which also travel in schools. Many pairs of eyes are much better than one at spotting the prey.
Anyone who has seen or swum with a school of fish knows how quickly they can change direction and shape of the school, and how amazingly well they keep together. Fish learn to do this when young, by first swimming in pairs and then in larger and larger groups as they improve their techniques and as their senses mature. Most fish do not learn how to school from the older fish, and its development is an instinctive behaviour pattern. One exception to this is the fierce predator, the piranha, which is born into, and lives its life in, a school.
The fish are able to keep together partly because their eyes are on the sides of their heads, which means they can see their neighbours easily, and can easily detect any changes in direction. Many fish, especially those living in murky waters, also have lateral lines that detect movements and displacement of water, and this helps in keeping the school together.
A school may seem harmonious, but there is a lot of competition for the safest spots in the centre of the school, farther away from the teeth of predators.
In many ways schools of fish resemble flocks of birds or herds of land animals, and they stick together for the same sorts of reasons. There are times when people seem to form ‘schools’ or ‘flocks’ too. Remember what your mother told you? There’s safety in numbers.