Behavior pattern (innate action) of individuals in fish schools generating efficient collective evasion from predation.

The schooling of fishes is one typical animal social behavior. One primary function of fish school is to protect members when attacked by predators. One main way that the school reduces the predator's chance of making a successful kill is to confuse the predator as it makes its strike. This may be accomplished by collective evasion behaviors organized through integration of motions of individual fish made based on their innate actions (behavior patterns). In order to investigate what kind of behavior pattern of individuals can generate the efficient collective evasion of a school, we present a model of evasion behavior pattern which consists of three component behavior patterns, schooling, cooperative escape, and selfish escape behavior patterns and the rule for choice of one among them with proper timing. Each fish determines its movement direction taking into account simultaneously three kinds of elemental motions, mimicking its neighbors, avoiding collisions with its nearest neighbors, and escaping from an approaching predator. The weights of three elemental motions are changed depending on which component behavior pattern the fish carries out. The values of the weights for three component behavior patterns can be definitively determined under the condition that the collective evasion of the school becomes the most efficient, that is, the probability that any member is eaten by the predator becomes minimum.