Absorption of acoustic energy in dense herring schools studied by the attenuation in the bottom echo signal

Abstract

The absorption of acoustic energy in herring schools is studied by echo integration of the bottom signal under various densities of herring. The investigations were carried out in a fjord in northern Norway, where large schools of adult herring overwinter. The intensity of the bottom signal, expressed by the integrator value (reflected area in square meters per square nautical mile(m^2/NM^2 )) of the bottom decreased significantly as the density of herring increased. The bottom integrator value decreased exponentially from 1.39 to 0.36 · 10^6 as the area density of herring increased from 0 to 550 · 10^3 corresponding to an increase in number of herring per square meter of 0 - 180. A method for correction of this effect is also presented.