Acoustic resonance by fish schools. A proposal for the schooling mechanism
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.