Fish waves as emergent collective antipredator behavior

The collective behavior of animals has attracted considerable attention in recent years, with many studies exploring how local interactions between individuals can give rise to global group properties.1–3 The functional aspects of collective behavior are less well studied, especially in the field,4 and relatively few studies have investigated the adaptive benefits of collective behavior in situations where prey are attacked by predators.5,6 This paucity of studies is unsurprising because predator-prey interactions in the field are difficult to observe. Furthermore, the focus in recent studies on predator-prey interactions has been on the collective behavior of the prey7–10 rather than on the behavior of the predator (but see Ioannou et al.11 and Handegard et al.12). Here we present a field study that investigated the anti-predator benefits of waves produced by fish at the water surface when diving down collectively in response to attacks of avian predators. Fish engaged in surface waves that were highly conspicuous, repetitive, and rhythmic involving many thousands of individuals for up to 2 min. Experimentally induced fish waves doubled the time birds waited until their next attack, therefore substantially reducing attack frequency. In one avian predator, capture probability, too, decreased with wave number and birds switched perches in response to wave displays more often than in control treatments, suggesting that they directed their attacks elsewhere. Taken together, these results support an anti-predator function of fish waves. The attack delay could be a result of a confusion effect or a consequence of waves acting as a perception advertisement, which requires further exploration. •Field experiments investigated anti-predator benefits of fish collective behavior•Fish produced conspicuous, repetitive, and rhythmic surface waves for up to 2 min•Experimentally induced fish waves reduced attack frequency in predatory birds•The results further support an anti-predator function of fish collective behavior Doran, Bierbach, Lukas et al. demonstrate anti-predator benefits of collective behavior in fish in the wild. In response to bird predation, fish produce conspicuous and repetitive surface waves for up to 2 min. These waves reduce predator attack rates and, in one predator species, attack success probability, showing the adaptive benefit of collective behavior.