The effect of attenuation from fish on long-range active and passive acoustic sensing in the ocean

Attenuation from fish can reduce the intensity of acoustic signals and significantly decrease detection range for active and passive sensing in the ocean. Formulations for predicting attenuation from fish depend on the accurate characterization of population density and spatial distribution of fish groups along long-range propagation paths, which is difficult to achieve using conventional survey methods. Here, Ocean Acoustic Waveguide Remote Sensing (OAWRS) is used to instantaneously image massive Norwegian herring shoals that stretch for thousands of square kilometers and simultaneously measure attenuation from these shoals within the active OAWRS transmissions, as well as attenuation to ship-radiated tonals. Reductions in signal intensity are predicted using a normal-mode-based analytical theory for acoustic propagation through inhomogeneities in an ocean waveguide. The predictions of the waveguide attenuation formulation are in agreement with measured reductions, where the position, size, and population density of the fish groups are characterized using OAWRS imagery as well as in situ echosounder measurements of the specific shoals occluding the propagation path. Negligible attenuation was observed in previous OAWRS surveys because the frequency of the acoustic signals was sufficiently far from the swimbladder resonance peak of the shoaling fish or the packing densities of the shoals were not sufficiently high.