Low-frequency acoustical interactions and scattering from schools of swim bladder fish.

Abstract

Low-frequency acoustic scattering from swim bladder fish is dominated by the bladder resonance response. Dense schools of these fish frequently consist of individuals of similar size, arranging themselves about one fish length apart. At near-resonance frequencies, acoustical interactions between the fish can cause the ensemble behavior of the school to become highly complex. Multiple scattering between neighboring fish affects the wave field of each individual. Additionally, since the wavelength at resonance is generally many times the fish spacing, the scattered fields interact coherently. Both features must also be incorporated to realistically describe scattering from fish schools. An effective methodology is available through the application of self-consistent multiple scattering techniques. The mathematical formalism used is based upon the solution of sets of coupled differential equations and incorporates a verified swim bladder scattering kernel for an individual fish. All orders of multiple scattering interactions between the fish are included, and the aggregate scattering field is calculated by coherent summation. Application to groups of closely spaced fish indicates significant deviations from the returns expected when incoherent scattering is dominant. The approach promotes an improved understanding of the ensemble physical properties of, as well as backscattering from, schools of swim bladder fish.