The level of order in animal groups on the move can display a wide range of variations, from fully disorganized aggregates to regular networks. This collective motion could not be achieved if individuals were assessing their environment independently; instead, it emerges due to interactions within the group. Ability to organize thus strongly depends on disturbance of the environment, which can alter how group members perceive their neighbors. For fish schools, information collection relies on sensory mechanisms, namely vision and flow sensing. However, quantitative description of sensory thresholds leading to schooling state transition is still lacking. We show that the group structure of rummy-nose tetra (Hemigrammus rhodostomus) exhibits distinct dynamics depending on the illumination of their habitat. Free swimming assemblies of ca. 50 fish are recorded in a large tank, where illuminance level is adjusted from 0 to 350 lux. We quantify geometrical order with a milling parameter, which capture global rotation: for low light exposure, we observe little to no order; intensity of the collective motion then increases continuously with illumination. Our data suggest that vision capability plays a major role in the level of order of a fish school.
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