noun - Bait ball, or baitball is a tightly packed formation of small fish that serves as a defensive measure used by small schooling fish when they are threatened by predators. Small schooling fish, including sardines and horse-eye jacks (shown on right) are eaten by many types of predators from sharks to whales.
The formation of a baitball is instinctual behavior as a defense mechanism because single fish are more likely to be eaten compared with large groups. The bait balls are short lived and seldom last longer than 10 minutes. But bait balls are conspicuous and draw the attention of predators. In response to the defensive capabilities of schooling fish, some predators use sophisticated countermeasures. These countermeasures can be spectacularly successful, and can seriously undermine the defensive value of forming bait balls.