Broad bandwidth acoustic backscattering from sandeel—measurements and finite element simulations

Acoustic backscattering measurements were made of individual sandeel (Ammodytes marinus L.) at frequencies from 100 to 280 kHz in a laboratory tank. Measurements were made versus incidence angle, −30° to 30° relative to both dorsal and lateral aspect directions, for dead whole sandeel and for excised backbones. Backscatter spectra from whole fish have deep minima at frequencies that vary with fish length and shift to lower frequencies at higher incidence angles. The backscatter spectra from dorsal and lateral aspects differ both in amplitude and in positions of the minima. The backscatter from the excised backbone increases almost linearly with frequency and, below about 150 kHz, contributes only a minor amount to the total sandeel backscatter. A finite element model of sandeel backscatter, developed and compared with the measurements, indicates that shear waves in the backbone contribute little to the overall backscatter from sandeel.