The definition of a shoal is a 'group of fish which remain together for social reasons, including feeding, reproduction or resting and can comprise of only a few fish to millions in the open sea'.

In every day conversation we often class shoaling and schooling as one behavioural trait. This is not the case, although both mean that the fish stay in one group, they do have different characteristics in the way they form the shoal or school.

Shoals contain fish which swim together with no formation pattern or in any synchronised way. They have a varying distance between them and their neighbouring fish. Schools, on the other hand contain fish which swim in an organised and planned way. They tend to swim at the same speed and keep close together. Shoals of fish that are on the move almost always form schools.

ADVANTAGES

The main advantage of shoaling is that of protection from predators. The phrase 'two heads are better than one' is well known and applies in this case. Except it is more likely to be 'hundreds of heads are better than one'. Shoaling multiplies the number of eyes and the directions in which they are looking for predators. This increases the probability of a predator being seen before it attacks. Fish alert each other by moving in a certain way. For example, the European minnow performs a movement called the 'skitter' A skittering fish alerts the other members of the shoal that danger has been spotted. Co-ordinated movements of shoals are thought to deter and even in some cases prevent predator fish from attacking if the shoal is large enough.
When attacked, fish have many co-ordinated manoeuvres that they may perform. Some fish simply perform what is known as a 'U-turn' - this means they divide into two and pass the predator on either side. This is performed in the hope the predator will be confused by the fact his target has split and passed him on either side. Another escape response it to divide in half, again, and to swim off in opposite directions. This movement is also hoped to confuse the predator by the fact that he will watch his intended target move in opposite directions and become confused. Perhaps the most attractive movement is for the shoal to 'explode'. When fish 'explode' they all try to escape in opposite directions. However, if the shoal is not quick enough to regroup they face the danger of being picked off slowly. The last of the manoeuvres is for fish to form into a tight ball. However, may people consider this to be a 'selfish' act by the shoal as only the fish in the middle are being protected by those on the outsides.

Shoaling also increases the group's chance of successful migration. Many species migrate and experiments have proved that individuals are far less successful in negotiations than large shoals. Another advantage to fish when they shoal is that it is thought that as a group they may be able to reduce drag and therefore be able to swim more efficiently. I say that this is a thing which is 'considered' an advantage because there has actually been no experimental proof. However, it works in theory.

DISADVANTAGES

Living in a large group means that there is heavy competition for food. Living in a shoal means that resources have to be stretched to suit everyone and when such a situation occurs there in inevitable tension.
Shoaling is thought to lessen the chance of being eaten. It is true that a predator will take longer to find one big group than many scattered individuals. However, once a predator has found a fish it has the same probability of being eaten whether it is in a large shoal or on its own.

SHOALING IN A TANK

You would expect fish to stop shoaling when in a tank because it is largely a predator free zone. It is common knowledge, though, that fish are stupid and they don't know if there isn't a predator in the tank. They are always expecting to find a predator waiting behind the next leaf! So, if a fish is classed as a shoaling fish it is best to keep them in a shoal even in a tank, because without other members of its species it will feel vulnerable and will become stressed due to this. Fish have been known to group with other species, for example a Guppy may shoal with a minnow. But given the choice, a fish will always shoal with its own kind.