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Predation has long been considered a powerful selective force governing the evolution and maintenance of gregariousness. Membership in a stable aggregation may afford the individual with a reduction in the probability of being successfully attacked, when compared with isolated individuals. This mitigation of predation accrues passively through the effects of dilution and confusion, and actively through the differential value of spatial position and the evolution of group avoidance manoeuvres. The relative safety of a given spatial position can change drastically, depending on attack strategy. Aquatic vertebrate predators (fish, birds, and marine mammals) display a wide variety of strategies when attacking schooling prey, ranging from marginal and invasive to exploitative. Evolution of avoidance manoeuvres by the school is dependent on the individual correctly assessing the potential for danger and responding in a timely, coordinated fashion. Whether safety accrues differentially among school members, and between school members and isolates, depends on: the size and shape of the school, the individual member's behaviour, the 'behaviour' of the group, and, most importantly, the relative frequency and abundance of each predator type.
Julia K. Parrish (Tue,) studied this question.