Concept
A bait ball, or baitball, occurs when small fish swarm in a tightly packed spherical formation about a common centre. It is a last-ditch defensive measure adopted by small schooling fish when they are threatened by predators. Small schooling fish are eaten by many types of predators, and for this reason they are called bait fish or forage fish. For example, sardines group together when they are threatened. This instinctual behaviour is a defence mechanism, as lone individuals are more likely to be eaten than an individual in a large group. Sardine bait balls can be in diameter and extend to a depth of . The bait balls are short-lived and seldom last longer than 10 minutes. However, bait balls are also conspicuous, and when schooling fish form a bait ball, they can draw the attention of many other predators. As a response to the defensive capabilities of schooling fish, some predators have developed sophisticated countermeasures. These countermeasures can be spectacularly successful, and can seriously undermine the defensive value of forming bait balls. Shoaling and schooling Small pelagic fish live in the open water, so unlike demersal or reef fish, they cannot hide among kelp, or in crevices in coral, or under rocks on the bottom. This leaves them vulnerable to attack by large predatory fish, as well as other predators, such as marine mammals and seabirds. As a result, small pelagic fish usually aggregate in schools for protection. Schooling fish have evolved sophisticated evasion techniques. When they school, they have many eyes, making ambush difficult; and their silvery bodies dazzle, making it difficult for predators to pick out individual fish. They react to movements from a predator with lightning reflexes, rhythmically streaming up and down with rapid direction changes. When a predator approaches, they can split and reform behind the predator. During the final stages of an attack, they can explosively disband in all directions and then just as rapidly reform. Defensive manoeuvres like these appear to be choreographed, though they are not.
