Better and faster collective decisions by larger fish shoals in the wild

Studies on collective cognition have provided many examples of decision-making benefits in terms of animals sharing information about predators, prey or resources in their environment. It has been shown how the efficient spread of adaptive information within groups can pro-vide benefits which increase with group size. Little is known, however, to which extent groups also amplify maladaptive information such as false alarms and whether such costs reduce or even nullify the above benefits. Here, we investigated fish shoals in the wild that responded collectively with escape dives when attacked by birds. We analysed the response of shoals in reaction to hard-to-detect bird attacks and similar but harmless flybys as a func-tion of shoal size. With increasing shoal size fish increasingly detected predator attacks (true positives) while their false alarms remained constant. Therefore, larger shoals became better at correctly classifying potentially dangerous stimuli rather than becoming more sensitive to all stimuli potentially related to attacks. In addition, decision time decreased with increasing shoal size. Larger shoals were thus able to mitigate two major trade-offs inherent in solitary decision making: the trade-off between true and false positives and the trade-off between speed and accuracy. We report performance increases at shoal sizes of tens of thousands of fish and pose challenges for the modelling of the underlying mechanisms.