A bait ball, or baitball, occurs when small fish swarm in a tightly packed spherical formation about a common center. It is a last-ditch defensive measure adopted by small schooling fish when they are threatened by predators. For example, sardines group together when they are threatened. This instinctual behavior is a defense mechanism, as lone individuals are more likely to be eaten than large groups. Sardine bait balls can be 10–20 meters in diameter and extend to a depth of 10 meters. The bait balls are short lived and seldom last longer than 10 minutes. Note that bait balls are also easy for predators to see, and when schooling fish form a bait ball they can draw the attention of many other predators.