Bait ball

A bait ball, or baitball, occurs when small fish swarm in a tightly packed spherical formation about a common centre.^[1] 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 10–20 metres (33–66 ft) in diameter and extend to a depth of 10 metres (33 ft). 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.