Turbidity increases risk perception but constrains collective behaviour during foraging by fish shoals

Turbidity reduces the distance that animals can detect food, predators and conspecifics. How turbidity affects decision making in social contexts has rarely been investigated; moreover, it is unknown whether decreased shoaling in turbid water is due to visual constraints (a mechanistic explanation) or a reduced perception of predation risk (an adaptive explanation). Using a V-shaped decision-making arena, we investigated the effect of turbidity on foraging in groups of three-spined sticklebacks, Gasterosteus aculeatus. In turbid conditions, fish took longer to leave a refuge and locate the food in one of the arms and consumed less food once it was found. This increase in risk-averse behaviour was further supported by improved accuracy over repeated trials and a speed–accuracy trade-off only being observed in turbid conditions. Despite evidence of a higher perception of risk in turbid water, the first fish to choose an arm of the maze was more likely to be alone in turbid water; thus, this individual lost the antipredator and decision-making benefits of collective behaviour. This suggests that turbidity acts mechanistically as a visual constraint, shifting decisions away from being made collectively to being made by individuals separated from the group, which could have potential impacts for wild prey populations.