  We report on the dynamics of collective alignment in groups of the cichlid
fish, Etroplus suratensis. Focusing on small-to-intermediate sized groups
($10<N<100$), we demonstrate that schooling (highly polarised and coherent
motion) is noise-induced, arising from the intrinsic stochasticity associated
with finite numbers of interacting fish. The fewer the fish, the greater the
(multiplicative) noise and therefore the likelihood of alignment. Such
empirical evidence is rare, and tightly constrains the possible underlying
interactions between fish: computer simulations indicate that E. suratensis
align with each other one at a time, which is at odds with the canonical
mechanism of collective alignment, local direction-averaging. More broadly, our
results confirm that, rather than simply obscuring otherwise deterministic
dynamics, noise is fundamental to the characterisation of emergent collective
behaviours, suggesting a need to re-appraise aspects of both collective motion
and behavioural inference.
