Field measurements of acoustic absorption in seawater from 38 to 360 kHz.

Accurate estimates of acoustic absorption in seawater are crucial to the acoustic estimation of aquatic biomass. Estimates of acoustic absorption were obtained via a "pulse-echo" method, implemented using commonly available scientific echosounders and spherical calibration targets over a range of discrete frequencies. Below about 200 kHz, the absorption estimates were not significantly different from those of existing formulas, but at around 333 kHz, the measured absorption was 15 dB km-1 higher than estimated from existing formulas. Measurement variability was about ±2 dB km-1 for all frequencies. This is consistent with an observed anomaly between modelled and measured frequency-dependent biological backscatter. Allowing for this deviation will avoid incorrect spectral-based classification of acoustic targets and improve uncertainty in aquatic biomass estimation.