Fish swim in shoals or schools for many reasons, one of it is to protect themselves against predators by swimming in a large group. But what happens when a predator attacks a group of fish? Let’s find out:
Synchronise their movements
Fish in shoal usually swim in different directions but when predators attack, they will shift to schooling tactic by being closer to other fish and swim in parallel with each other in the same direction.
Some members of the shoal may approach the predator and then return to the shoal. They do this to assess the identity or motivational state of the predator, to show the predator that it has lost the element of surprise. This is seen in guppies, stickleback and gobies.
Some groups can deter predators by acting aggressively. This is documented in squirrelfishes, snappers, grunts, damselfishes. They may make contact or display their spines.
Alarm reactions (or fright response)
An alarm substance is released when the skin of the fish is broken during a predatory attack. Reactions to the alarming substance can differ. Fish mostly react to the substance by schooling tightly and moving away while some fish freeze or use colouration to avoid detection.
How predators make use of groups of prey
Despite the shoaling or schooling fish’s defences, predators also have tricks up their fins.
A major problem when attacking prey in groups is the presence of other group members. So a predator would try to separate a few prey from the group and attack them. Stragglers are 50 times more likely to be attacked than fish in a group.
Larger predators may hide itself among groups of fish so it can feed on its prey, such as in the case of the trumpet fish and its prey, the damselfish.
Although some predators might stand out in a group of other fish, the disadvantage outweighs the advantages of gaining more access to resources (food).
Come visit S.E.A. Aquarium and see the schooling fish in action.