The roles of density, geometry, and short-range positional order in low-frequency acoustic scattering from fish schools

Fish can be efficient scatterers of low-frequency underwater sound. This results from the large response of their swim bladders to incoming acoustic waves at frequencies close to their natural resonance frequencies. For large and densely packed fish schools the scattering returns deviate significantly from what could be expected by adding the individual fish returns. These deviations are caused by multiple scattering between the individual fish in the school and concern the target strength as well as the spectral and directional characteristics of the scattering cross sections. Acoustically, a fish school can be viewed as a single object with acoustic bulk parameters determined from the properties of the individual fish and their positional arrangement in the school. This effective medium approach incorporates the acoustic coupling of all fish due to multiple scattering and works for a wide range of fish sizes, -numbers, and -densities. In this study, the effective medium approach as well as numerical computations from first physical principles are employed to investigate the coherent and diffusive low-frequency scattering cross sections of large and dense fish schools as a function of fish density, school geometry, and positional order among the individual fish. Inversions for these parameters are explored.