BIOLOGICAL FLUID DYNAMICS
Random patterns in fish schooling enhance alertness
One of the most highly debated questions in the field of animal swarming and social behaviour, is the collective random patterns and chaotic behaviour formed by some animal species, in particular if there is a danger. Is such a behaviour beneficial or unfavourable for survival? Here we report on one of the most remarkable forms of animal swarming and social behaviour - fish schooling - from a hydrodynamic point of view. We found that some fish species do not have preferred orientation and they swarm in a random pattern mode, despite the excess of energy consumed. Our analyses, which includes calculations of the hydrodynamic forces between slender bodies, show that such a behaviour enhances the transfer of hydrodynamic infor- mation, and thus enhances the survivability of the school. These findings support the general hypothesis that a disordered and non-trivial collective behaviour of in- dividuals within a nonlinear dynamical system is essential for optimising transfer of information - an optimisation that might be crucial for survival.
The forces and moments on a body immersed in incom- pressible fluid resulting from the hydrodynamic interactions induced by the movement of a second body are strongly dependent on the lateral separation distance. We calculate the forces on two submerged bodies moving together at different longitudinal distances between bows, building up a quasi-steady solution for the forces during an overtaking manoeuvre.
Applying the second-order approximation provides much better qualitative and quantitative agreement with the experimental measurements carried out by Newton, as well as with the numerical results of the boundary element method presented by Newton [DTMB Report 1461, Washington, D.C., pp 1–24 (1960)], solving previous discrepancies. Carrying out even higher order potential approximations is probably not very useful, as the neglected viscous effects become more important.
