Shoal cohesion and polarisation increase in turbid water and improve foraging performance

Group living can make animals more resilient or more susceptible to human-induced rapid environmental change, and whether animals demonstrate plasticity in their collective behaviour to mitigate the effects of environmental change is unknown. Freshwater and coastal habitats are becoming increasingly turbid as a result of human activity, which limits the availability of information and hence responses to critical stimuli such as food and predators. Here, we tested whether the well-documented improvement in foraging performance by larger groups is affected by moderate levels of water turbidity. Fish shoals were faster to attack the food stimulus in larger groups and slower in turbid water, effects which were independent of one another. However, shoals became more cohesive and coordinated (i.e. more polarised in direction) in turbid water, and this corresponded to more cohesive and polarised shoals being faster to attack the food stimulus in turbid water but slower in clear water. Our results demonstrate that fish shoals can increase cohesion and coordination in moderate levels of turbidity, that this plasticity improves their response to ecologically relevant ephemeral stimuli, and that collective behaviour, rather than group size, can mediate the response of groups to environmental change.