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 forwa rd 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. Comp arison with the PC-DV;BA (Phase-Compensated-Distorted Wave Born Approximati on) shows a reasonable agreement between the two models. The extinction cro ss section is shown to be proportional to (ka)(2) and the induced attenuati on is predicted for various species of zooplankton over a wide frequency ra nge. It is shown that, under certain realistic conditions, such as the pres ence of a swarm of aggregated krill, sound extinction by zooplankton could influence the acoustic measurements significantly, as much as an 85% reduct ion in acoustic intensity for Euphausia superba with n = 5000 ind./m(3) ove r a range of 50 m. (C) 2001 Acoustical Society of America.