  Crowd movements are observed among different species and on different scales,
from insects to mammals, as well as in non-cognitive systems, such as motile
cells. When forced to escape through a narrow opening, most terrestrial animals
behave like granular materials and clogging events decrease the efficiency of
the evacuation. Here, we explore the evacuation behavior of macroscopic,
aquatic agents, neon fish, and challenge their gregarious behavior by forcing
the school through a constricted passage. Using a statistical analysis method
developed for granular matter and applied to crowd evacuation, our results
clearly show that, unlike crowds of people or herds of sheep, no clogging
occurs at the bottleneck. The fish do not collide and wait for a minimum
waiting time between two successive exits, while respecting a social distance.
When the constriction becomes similar to or smaller than their social distance,
the individual domains defined by this cognitive distance are deformed and fish
density increases. We show that the current of escaping fish behaves like a set
of deformable 2D-bubbles, their 2D domain, passing through a constriction.
Schools of fish show that, by respecting social rules, a crowd of individuals
can evacuate without clogging, even in an emergency situation.
