Defensive strategies of schooling prey under predatory stress

The strategy and tactics of avoiding a predator by a schooling prey are examined using the specific example of rainbow trout ( Salmo qairdneri ) chasing single and schooling sockeye salmon ( Oncorhyncus nerka ). Three general rules of defensive strategy are developed from the examination of predation as a process and from parallels in modern aerial warfare. The first rule is based on prey vigilance. Rule 1. The best strategy from the prey point of view is not to be detected by a predator, and to detect the presence of a predator as soon as possible, preferably before detection. It is best to avoid a chase, which can be achieved by hiding, or moving away, so as to increase the distance from the predator. The second rule is based on group cohesion. Rule 2. Individuals and strays from groups are more vulnerable to predators, and school size and structure is limited by signal loss between individuals, therefore, the group should become more compact in spacing when attacked by predators. This will allow the execution of group manoeuvres with minimal group disintegration. The third rule is based on tactical manoeuvre considerations. Rule 3. The best manoeuvre that the group can perform, if it detects the predator at a distance that enables the manoeuvre to be executed, is to turn toward the direction of the predator. This enables the individuals to move around the predator when it engages the group. This results in positioning the predator behind and heading away from the group. The predator was found to use path prediction as an interception strategy and prey used rapid turning manoeuvres as a defensive strategy. The first response of schooling prey was to move away from the path direction of the predator while forming a more compact school. The second response of the school was to turn toward the path of the predator. The third response was rapid school disintegration as each individual turned rapidly and accelerated to a high linear velocity and oscillating angular velocity. Schooling by prey confuses and limits the ability of the predator to path predict. Consequently, predator capture success is greater when chasing single prey.Science, Faculty ofZoology, Department ofGraduat