  The acoustic properties of a fish school have been modelled using a cloud of
bubbles. Similar to how bubble clouds present a collective monopole mode, a
fish school also shows a collective breathing mode in which the whole school
resonates as a single body. Here, we conjecture that the underwater acoustic
noise of the ocean amplified by the multiple scattering of swim bladders in the
fish school might produce attractive acoustic forces that are strong enough to
account for the schooling mechanism. Our model predicts the presence of
attractive Bjerknes forces, as large as 30% of the fish weight for 20-metre
large fish schools, and that the attractive force on every fish in the centre
of the school is cancelled when the fish increase/decrease the volume/pressure
of its gas bladder. To the best of our knowledge, this is the first example of
a purely mechanical force that might contribute to fish being bound to or
released from the school. The study may lead to new areas of research in many
scientific fields beyond the nearest-neighbour interaction mechanism
customarily used, and it would help in our understanding of collective
processes of systems in ecology, physics, chemistry, and biology.
