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It is well known that the acoustic estimation of fish abundance requires an estimate of the absorption of sound in seawater. Any errors in this factor will lead to errors in abundance estimates that increase with range. Concerns over the accuracy of the widely used relationship of Francois and Garrison (Journal of the Acoustical Society of America, 72: 896–907; Journal of the Acoustical Society of America, 72: 1879–1890) led to a new analysis of the data they used and a reconsideration of the relationship as a whole. A major component of sound absorption by seawater is the relaxation frequency of MgSO4, for which there are at least five equations in the literature. We review these and our analysis uses the relaxation equation that best fits the sound-absorption data using modern regression methods. We propose a new equation for the range of frequencies used in fish abundance (10–120 kHz) based on this analysis. For a typical New Zealand deep-water fishery carried out at 38 kHz the new equation gives a biomass 17% lower than that yielded by Francois and Garrison. Whilst we believe that the new equation gives more accurate results for the range of frequencies used in fish abundance, there remains much uncertainty which will only be resolved by the collection of new in situ sound-absorption data in this frequency range.

The absorption of sound in seawater in relation to the estimation of deep-water fish biomass