Effect of orientation of euphausiids and copepods on acoustic target strength: Implications for measurements from down-looking and side-looking acoustic systems

• Published in: The Journal of the Acoustical Society of America 2004-05, Vol.115 (5), p.2583-2583
• Main Authors: Sutor, M, Cowles, T J
• Format: Article
• Language: English
• Subjects: Copepoda; Euphausiacea; Marine
• ISSN: 0001-4966; 1520-8524
• DOI: 10.1121/1.4784339

Multifrequency acoustics is a potentially useful tool for zooplankton ecologists to rapidly map distributional patterns and determine taxonomic and size composition of scatterers. This is typically done with down-looking or side-looking acoustic systems. The ability to estimate taxonomic and size composition acoustically depends upon accurate models of scattering from zooplankton. In this study, distorted wave Born approximation (DWBA) models of individual zooplankton were used to illustrate that the frequency dependence of expected scattering is different for down-looking and side-looking acoustic systems due to the differences in orientation of scatters within the sampled volume. The results show that peaks and nulls occur at different frequencies for each system with maximum target strength differences of 30 dB. The models were used to predict volume scattering (S sub(V)) based on zooplankton collected from MOCNESS tows. Predicted S sub(V) was compared to S sub(V) measured by both a down-looking HTI acoustic system and a side-looking TAPS. The results showed that changes in orientation can have large effects on predicted total S sub(V), particularly at higher frequencies, and demonstrate that choice of orientation parameters is important and different parameters should be used when comparing predicted S sub(V) with measured S sub(V) from down-looking or side-looking acoustic systems.