Plasticity in collective behaviour helps fish shoals mitigate limited visual information from water turbidity

Group living can make animals more resilient or more susceptible to human-induced rapid environmental change. Freshwater and coastal habitats are becoming increasingly turbid because of human activity, which limits the availability of visual information and hence responses to critical stimuli such as food and predators. Using shoals of three-spined sticklebacks (Gasterosteus aculeatus) responding to an unpredictable food stimulus, we confirm that fish were slower to attack in turbid water and were faster in larger groups. Turbidity and group size also modulated the effect of the shoals' collective behaviour: if small groups were in turbid water, the latency to attack was faster when they were more cohesive and more polarized, but in clear water, cohesion and polarization slowed the response of larger groups. These varying effects of collective state corresponded with differences before the food stimulus appeared, with fish increasing cohesion and coordination by being more polarized in direction, and being quicker to respond to one another's changes in speed in turbid water, especially in small groups. As the visibility of neighbours decreases in turbid water and should constrain collective behaviour, enhanced collective behaviour is instead consistent with an adaptive, plastic response that improves responses to stimuli when visual information is limited.