Swarm location in zooplankton as an anti-predator defence mechanism

Abstract. The hypothesis that predators are less willing to attack dense swarms when far from cover was tested. Transparent cell compartments containing different densities of the prey, Daphnia magna, were placed either near to or far from a refuge for the predator, and two either hungry or satiated three-spined sticklebacks, Gasterosteus aculeatus, were allowed to attack the cell compartment. The trade-off between feeding maximization and predator avoidance in sticklebacks varied, depending on distance to cover, hunger level and the distance to the other fish. Hungry fish and the satiated fish in the safe areas were willing to risk feeding in confusing densities of prey (as in swarms), while satiated fish far from the cover were not. Hence, zooplankton swarms would benefit from swarming in areas where planktivorous fish are at most risk from predators.