  We explore the collective behaviours of 7 group sizes: 1, 2, 3, 5, 7, 10 and
20 AB zebrafish (Danio rerio) in a constraint environment composed of two
identical squared rooms connected by a corridor. This simple set-up is similar
to a natural patchy environment. We track the positions and the identities of
the fish and compute the metrics at the group and at the individual levels.
First, we show that the size of the population affects the behaviour of each
individual in a group, the cohesion of the groups, the preferential
interactions and the transition dynamics between the two rooms. Second, during
collective departures, we show that the rankings of exit correspond to the
topological organisations of the fish prior to their collective departure with
no leadership. This spatial organisation emerge in the group a few seconds
before a collective departure. These results provide new evidences on the
spatial organisation of the groups and the effect of the population size on
individual and collective behaviours in a patchy environment.
