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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 defense mechanism, as lone individuals are more likely to be eaten than large groups. Sardine bait balls can be 10–20 metres in diameter and extend to a depth of 10 metres. 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. ==Background== 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 together in schools for protection. Schooling fish have evolved sophisticated evasion techniques. When they school they have many eyes, which makes ambush difficult, and their silvery bodies dazzle, which makes it difficult for predators to pick out individual fish.〔Megurran, AE (1990) ("The adaptive significance of schooling as an anti-predator defense in fish" ) ''Annales Zooligici Fennici'', 27: 51-66.〕 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.〔Partridge BL (1982) ("The structure and function of fish schools" ) ''Scientific American'', 246(6) 114–123.〕〔Magurran AE and Pitcher TJ (1987) ("Provenance, shoal size and the sociobiology of predator-evasion behaviour in minnow shoals" ) ''Proc. R. Soc. Lond. B'', 229(1257): 439-465. 〕 Defensive manoeuvres like these appear to be choreographed, though they are not. Within the school itself, there is no centralized intelligence aware of how the school has configured itself. Rather, the schooling behaviour is the emergent consequence of relatively simple rules followed by each individual fish in the school, such as remaining close together, moving in the same direction, and avoiding collisions with each other.〔Parrish JK, Viscido SV and Grunbaumb D (2002) ("Self-organized fish schools: An examination of emergent properties" ) ''Biol. Bull.'' 202: 296 –305.〕 Some species of forage fish, driven by nutrient availability and their life-cycle stage, form vast schools at predictable locations and times of the year. Normally, schooling works well as protection from occasional predators. But when fish school in vast numbers they can attract correspondingly huge numbers of predators, including seabirds, sharks, tuna, billfish, pods of dolphins, and killer and humpback whales. Aggregations of predators on such a scale means the schools can be attacked on all sides, and panicked into forming bait balls.〔Seifert DD (2010) (Water Column: Finishing School ) November 2010, ''Dive Magazine UK''.〕 A bait ball is a last-ditch defensive measure adopted by fish schools when they are overwhelmed and more effective defensive strategies have broken down. Schooling fish are easier to attack once they abandon their free streaming behaviour and form into a tight bait ball. Many predator species have learned that by interacting cooperatively they can panic schooling fish into forming a bait ball. This cooperative behaviour can occur both intraspecifically (among the individuals within a predator species) and interspecifically (across individuals belonging to more than one predator species). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Bait ball」の詳細全文を読む スポンサード リンク
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