Some aspects of sound extinction by zooplankton

To address the importance of sound extinction (or the shadowing effect) in the presence of a densely aggregated zooplankton layer, acoustic scattering by weakly scattering fluid objects is studied theoretically. An analytical expression for the extinction cross section is obtained based on the forward scattering theorem and the analytical formula for the forward scattering amplitude is derived from the Modal Based-Deformed Cylinder Model (MB-DCM). The validity of the MB-DCM solution for the forward scattering amplitude and its sensitivity to geometrical and physical parameters is studied. Comparison with the PC-DWBA (Phase-Compensated-Distorted Wave Born Approximation) shows a reasonable agreement between the two models. The extinction cross section is shown to be proportional to (ka)2 and the induced attenuation is predicted for various species of zooplankton over a wide frequency range. It is shown that, under certain realistic conditions, such as the presence of a swarm of aggregated krill, sound extinction by zooplankton could influence the acoustic measurements significantly, as much as an 85% reduction in acoustic intensity for Euphausia superba with n = 5000 ind./m3 over a range of 50 m.