Limitations in measurements of acoustic scattering from fish at rather high frequencies

Recent years, the frequencies used in quantitative echo sounders have become multiplicative for the purposes of fish species identification, simultaneous observation of fish and plankton, etc. For example, frequencies of 18, 38, 70, 120, 200, and 364 kHz are used. At frequencies above 100 kHz, however, the absorption attenuation increases, and therefore the signal-to-noise ratio (SNR) becomes small and the complete correction of the attenuation is difficult to result in sometimes a large measurement error. This paper elucidates the SNR and the error and clarifies the limitations and cautions of the high frequency measurements. From the viewpoint of the SNR, a measurement of fish at 200 m depth is possible at up to 200 kHz, but a correction of the absorption becomes abruptly difficult above 70 kHz. Therefore fish scattering measurements at high frequencies need the absorption coefficients matched to environment.