  Collective behaviour in living systems is observed across many scales, from
bacteria to insects, to fish shoals. Zebrafish have emerged as a model system
amenable to laboratory study. Here we report a three-dimensional study of the
collective dynamics of fifty zebrafish. We observed the emergence of collective
behaviour changing between \yy{ordered} to randomised, upon \yy{adaptation} to
new environmental conditions. We quantify the spatial and temporal correlation
functions of the fish and identify two length scales, the persistence length
and the nearest neighbour distance, that capture the essence of the behavioural
changes. The ratio of the two length scales correlates robustly with the
polarisation of collective motion that we explain with a reductionist model of
self--propelled particles with alignment interactions.
