The low-frequency scattering action of fish schools

Abstract

Enhanced low-frequency scattering levels from a single fish at its swimbladder resonance and a frequency invariant return for an octave or so above have been predicted by simple monopole scattering models [Love (1977)]. These characteristics have also been measured, more or less, with fish schools [McCartney (1967)]. In this talk the ‘‘more or less’’ is explored by modeling the school as a number of monopole scatterers quasi-randomly distributed within an ellipsoidal volume, with and without mutual scattering taken into account. The swimbladder natural frequencies themselves are also taken to be random with a uniform PDF over a finite band. For a given realization, overall returns are computed in both the frequency and time domains. Computed fluctuations in aspect are compared with those for a Gaussian amplitude distribution. The influence of mutual scattering is shown as a function of the mean free path among individual fish. Finally, it is noted that though swimbladder gas volume and bulk modulus indicate a P5/6 hydrostatic pressure dependence, swimbladder natural frequencies typically vary as P1/2 [Weston (1967)]. [Work sponsored by Naval Undersea Warfare Center, New London Detachment.]