© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
The biological validation of ADCP acoustic backscatter through direct comparison with net samples and model predictions based on acoustic-scattering models
Southampton Oceanography Centre European Way, Southampton SO14 3ZH, UK
*Correspondence to S. Fielding: tel: +44 2380 596016; fax: +44 2380 596247. e-mail: sof{at}soc.soton.ac.uk.
Mean volume-backscattering strength (MVBS) data collected using a 153-kHz, narrowband Acoustic Doppler Current Profiler (ADCP) were compared with zooplankton abundance and biovolume data collected using a Longhurst–Hardy Plankton Recorder (LHPR). A direct comparison showed that there was a linear relationship between MVBS and log-transformed zooplankton dry weight. This linear relationship, determined from a mixed zooplankton-species population, was then compared with that reported in previous work from a region dominated by a single species of copepod and found to be significantly different. The scatter around the linear relationships determined between MVBS and log-transformed dry weights in regions of complex, mixed zooplankton populations results in our inability to distinguish different relationships that could be expected from different populations in varying oceanic regions. It is suggested that, without further manipulation of the data, ADCP MVBS cannot be used to determine quantitative estimates of zooplankton abundance and biomass in mixed populations.
"Observed MVBS" was compared with model-predicted backscattering, calculated using acoustic models and abundance and size measurements of zooplankton from net samples. The results show that at high backscattering intensities (>–80 dB) the observed MVBS from an ADCP was generally consistent with the model predictions. Abundance, biovolume, and model-predicted backscattering contributions of six "significant acoustic-scattering" groups (amphipods, chaetognaths, copepods, euphausiids, fish, and pteropods) are shown to vary disproportionately. In particular, a rare and small but strong acoustic scatterer such as a pteropod can contribute as little as 0.1% to the total sample abundance and 0.1% to the biovolume but represent 69.5% of the total model-predicted backscattering.
Model, instrumental, and methodological artefacts are identified as potential sources of inconsistencies between the observed and model-predicted backscattering. These include the effect of the orientation of zooplankton, inadequate knowledge of model parameters such as the sound-speed and density contrasts, the mismatch between sampling volumes of the net and acoustic instrument, and net avoidance by the more mobile scatterers.
Keywords: acoustic models, Arabian Sea, bioacoustics, Doppler sonar, MVBS, Northwest Indian Ocean, sound scattering, zooplankton
Received 7 May 2003; accepted 15 October 2003.
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