Effects of multiple stressors on fish shoal collective motion are independent and vary with shoaling metric

Collective movement is critical to the survival of some animals. Despite substantial progress in understanding animal collectives such as fish shoals and bird flocks, it is unknown how collective behaviour is affected by changes in multiple environmental conditions that can interact as stressors. Using a fully-factorial repeated-measures design, we test the independent and combined effects of darkness and acoustic noise on the collective motion of three-spined sticklebacks (Gasterosteus aculeatus) quantified using high-resolution tracking data from video. Corresponding to the importance of vision in shoaling behaviour, darkness increased nearest-neighbour distances and reduced coordination, measured as speed correlations and differences in directional heading between nearest neighbours. Although individual swimming speeds were not impacted by darkness, the group’s centre of mass was slower, an emergent effect of reduced polarisation (i.e. greater group disorder) in darkness. While additional acoustic noise had no detectable effect on these variables, it altered group structure, with fish being more likely to be found side-by-side one another. Fish were also further from the arena wall (i.e. showed reduced wall following) when there was additional acoustic noise. There was only weak evidence for additive or interactive effects of the two stressors. Across the different environmental contexts, there were consistent, repeatable differences between groups (i.e. group personality variation) in the speed, turning angle and distance from the arena wall of individuals, but the only collective behaviour that was repeatable was group polarisation. Our study demonstrates that multiple stressors can have independent effects that impact different aspects of behaviour and highlights the need for empirical studies on multiple stressors as their effects can be unpredictable.