Crowd movements can be observed across species and scales: from insects
to mammals but also in non-cognitive systems such as eukaryotic cells.
But, what happens when gregarious animals have to momentarily adopt an
individual behaviour? To challenge the balance between herd and
individual behaviours, a school of Neons (Paracheirodon innesi) was
forced to cross a constriction. This experiment is reminiscent of panic
crowd escaping through a narrow door. Using statistical analysis
developed for granular matter and applied to crowd evacuation, our
results clearly show that contrarily to human crowds or herds of sheep,
no clogging is formed at the bottleneck. Fish don’t collide and wait by
respecting their social distance and a minimum waiting time between two
successive exits. When the constriction starts to be comparable
to or smaller than their social distance, the individual domains fixed
by this cognitive distance must deform and the fish density is
increased. We show that the current of escaping fish indeed behaves as a
set of deformable 2D-bubbles, their 2D domain, through a constriction
and obeys the same law. The complex environment  has also been addressed
with an assembly of obstacles, preliminary results on this second
experiment will also be presented.