scholarly journals Modelling three-dimensional directivity of sound scattering by Antarctic krill: progress towards biomass estimation using multibeam sonar

2009 ◽  
Vol 66 (6) ◽  
pp. 1245-1251 ◽  
Author(s):  
George R. Cutter ◽  
Josiah S. Renfree ◽  
Martin J. Cox ◽  
Andrew S. Brierley ◽  
David A. Demer
Keyword(s):  
Antarctic Krill ◽  
Three Dimensional ◽  
Acoustic Scattering ◽  
Euphausia Superba ◽  
Target Strength ◽  
Biomass Estimation ◽  
Sound Scattering ◽  
Multibeam Sonar ◽  
Small Craft ◽  
Directivity Patterns

Abstract Cutter, G. R., Renfree, J. S., Cox, M. J., Brierley, A. S., and Demer, D. A. 2009. Modelling three-dimensional directivity of sound scattering by Antarctic krill: progress towards biomass estimation using multibeam sonar. – ICES Journal of Marine Science, 66: 1245–1251. Target strength (TS) estimation is a principal source of uncertainty in acoustic surveys of Antarctic krill (Euphausia superba). Although TS is strongly dependent on krill orientation, there is a paucity of information in this regard. This paper considers the potential for narrow-bandwidth, multibeam-echosounder (MBE) data to be used for estimating the orientations of krill beneath survey vessels. First, software was developed to predict MBE measurements of the directivity patterns of acoustic scattering from individual or aggregated krill in any orientation. Based on the distorted-wave, Born approximation model (DWBA), scattering intensities are predicted vs. MBE angles for specified distributions of krill orientations (pitch, roll, and yaw angles) and swarm densities. Results indicate that certain distributions of orientations, perhaps indicative of particular behaviour, should be apparent from the sonar data. The model results are compared with measurements on krill made using a 200-kHz MBE deployed from a small craft off Cape Shirreff, Livingston Island, Antarctica, in summer 2006. The stochastic DWBA model is then invoked to explain disparities between the model predictions and MBE measurements.

scholarly journals Validation of the stochastic distorted-wave Born approximation model with broad bandwidth total target strength measurements of Antarctic krill

2003 ◽  
Vol 60 (3) ◽  
pp. 625-635 ◽  
Author(s):  
David A. Demer ◽  
Stéphane G. Conti
Keyword(s):  
Born Approximation ◽  
Antarctic Krill ◽  
Target Strength ◽  
Biomass Estimation ◽  
Distorted Wave ◽  
Distorted Wave Born Approximation ◽  
Sound Scattering ◽  
Important Species ◽  
Field Station ◽  
Broad Bandwidth

Abstract Total-scattering cross-sections (σt) of Antarctic krill (Euphausia superba) were measured over a broad bandwidth (36–202 kHz) using a new technique based on acoustical reverberation in a cavity. From 18 February to 9 March 2002, mean total target strengths (TTS = 10 log(σt/4π)), were measured from groups of 57–1169 krill (average standard length=31.6 mm; standard deviation=6.6 mm) at the Cape Shirreff field station, Livingston Island, Antarctica, and aboard RV “Yuzhmorgeologiya”. Chirp pulses were transmitted sequentially by an omni-directional emitter into one of three glass carboys containing groups of krill swimming in 9.3, 19.3, or 45.9 liters of seawater (0.6°C≤temperature≤4.0°C). Between each pulse the krill moved within the fixed-boundary tank and the modulated reverberations were sensed bi-statically with three omni-directional receivers. At each center frequency (fc), the coherent energy in 200-pulse ensembles identified sound scattered by the tank. The incoherent energy described total sound scattering from the krill. Thus, the TTS at each fc was extracted from a correlation analysis of energy reverberated in the tank. Measurement bias was determined to be ±0.4 dB from an experiment using metal sphere reference targets, and the precision was estimated as ±0.8 dB from the variability in the krill TTS (fc) measurements. The empirical estimates of mean σt corroborated a krill-scattering model based on the distorted-wave Born approximation (DWBA), enhanced by the authors to account for the stochastic nature of sound scattering (SDWBA), integrated over all scattering angles and averaged over all incident angles (SDWBATTS). The SDWBA, solved for target strength of Antarctic krill, may be the best predictor of backscatter for this important species and may also provide backscattering spectra for improving their acoustic identification. These advances may help to reduce uncertainty in krill-biomass estimation using multi-frequency echosounder data and echo-integration methods.

scholarly journals Broad-bandwidth, sound scattering, and absorption from krill (Meganyctiphanes norvegica), mysids (Praunus flexuosus and Neomysis integer), and shrimp (Crangon crangon)

2005 ◽  
Vol 62 (5) ◽  
pp. 956-965 ◽  
Author(s):  
Stéphane G. Conti ◽  
David A. Demer ◽  
Andrew S. Brierley
Keyword(s):  
Antarctic Krill ◽  
Euphausia Superba ◽  
Target Strength ◽  
Distorted Wave ◽  
Crangon Crangon ◽  
Distorted Wave Born Approximation ◽  
Sound Scattering ◽  
Broad Bandwidth ◽  
Different Shapes ◽  
Meganyctiphanes Norvegica

Abstract Sound scattering and absorption by Northern krill (Meganyctiphanes norvegica) were measured over the acoustic bandwidth of 30–210 kHz and compared with similar scattering measurements for Antarctic krill (Euphausia superba). The measurements of total target strength (TTS; energy scattered in all directions, averaged over all angles of incidence) match the SDWBA model (stochastic distorted-wave Born approximation) recently developed for Antarctic krill, indicating its validity for other euphausiid species with similar size and shape. However, the TTS of crustaceans with markedly different shapes are not well predicted by SDWBA derived with the generic krill shape and scaled to animal length (L). Therefore, crustacean target strength (TS) may not be estimated accurately by a linear function of log10(L), irrespective of shape, questioning the validity of the current TS relationship used for Antarctic krill derived from data measured from multiple crustaceans. TTS and TS are dependent upon both L and shape, and different crustaceans have significantly different shapes and width-to-length relationships. In contrast, modelled TTS and TS spectra for gravid and non-gravid krill appear to have differing amplitudes, but similar shapes. Additionally, measurements of absorption spectra from decapods indicate that the absorption cross-section increases with the volume of the animal.

Three-dimensional observations of swarms of Antarctic krill (Euphausia superba) made using a multi-beam echosounder

2010 ◽  
Vol 57 (7-8) ◽  
pp. 508-518 ◽  
Author(s):  
Martin J. Cox ◽  
Joseph D. Warren ◽  
David A. Demer ◽  
George R. Cutter ◽  
Andrew S. Brierley
Keyword(s):  
Antarctic Krill ◽  
Three Dimensional ◽  
Euphausia Superba ◽  
Antarctic Krill Euphausia Superba

scholarly journals Combining multibeam-sonar and multifrequency-echosounder data: examples of the analysis and imaging of large euphausiid schools

2009 ◽  
Vol 66 (6) ◽  
pp. 991-997 ◽  
Author(s):  
Rolf J. Korneliussen ◽  
Yngve Heggelund ◽  
Inge K. Eliassen ◽  
Ola K. Øye ◽  
Tor Knutsen ◽  
...  
Keyword(s):  
Three Dimensional ◽  
Size Composition ◽  
Euphausia Superba ◽  
Size Distributions ◽  
Multibeam Sonar ◽  
Acoustic Data ◽  
Combining Data ◽  
Efficient Processing ◽  
Efficient Exploration ◽  
Different Sources

Abstract Korneliussen, R. J., Heggelund, Y., Eliassen, I. K., Øye, O. K., Knutsen, T., and Dalen, J. 2009. Combining multibeam-sonar and multifrequency-echosounder data: examples of the analysis and imaging of large euphausiid schools. – ICES Journal of Marine Science, 66: 991–997. The first high-resolution, quantitative, multibeam sonar (Simrad MS70) ever developed was mounted in a keel of RV “G. O. Sars” with port-orientated beams. Each ping samples a volume of 60° horizontally × 45° vertically with 500 beams, which is often enough to insonify a complete school of fish or zooplankton. The large amount of resulting data is efficiently preprocessed with automatic, real-time detections of school candidates; these are accepted or rejected during post-processing. The system was used on the continental shelf near the Subantarctic island of South Georgia to study Antarctic krill (Euphausia superba), and some of the detected schools were immediately sampled with a six-frequency echosounder (Simrad EK60), then trawled with various nets to verify the target species and their size composition. For schools acoustically categorized as euphausiids, data from the two acoustic systems were used to estimate the school morphometrics and the krill size distributions. The principal objectives of this study were to explore the potential of combining data from a multibeam sonar, multifrequency echosounders, and nets, and to describe the efficient processing methods and software that facilitate the multi-instrument analyses. Three-dimensional morphometrics based on the MS70 data were consistent with corresponding two-dimensional morphometrics based on the echosounder data and could be used to improve the acoustic classifications of taxa or species. Additionally, automatic preprocessing and integration of data from different sources into the same user interface allowed efficient exploration and interpretation of all the acoustic data.

scholarly journals Interannual variability in Antarctic krill (Euphausia superba) density at South Georgia, Southern Ocean: 1997–2013

2014 ◽  
Vol 71 (9) ◽  
pp. 2578-2588 ◽  
Author(s):  
Sophie Fielding ◽  
Jonathan L. Watkins ◽  
Philip N. Trathan ◽  
Peter Enderlein ◽  
Claire M. Waluda ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Interannual Variability ◽  
Antarctic Krill ◽  
Southern Oscillation ◽  
Euphausia Superba ◽  
Target Strength ◽  
South Georgia ◽  
Key Species ◽  
The Mean ◽  
Antarctic Krill Euphausia Superba

Abstract Antarctic krill (Euphausia superba) are a key species in Southern Ocean ecosystems, maintaining very large numbers of predators, and fluctuations in their abundance can affect the overall structure and functioning of the ecosystems. The interannual variability in the abundance and biomass of krill was examined using a 17-year time-series of acoustic observations undertaken in the Western Core Box (WCB) survey area to the northwest of South Georgia, Southern Ocean. Krill targets were identified in acoustic data using a multifrequency identification window and converted to krill density using the Stochastic Distorted-Wave Born Approximation target strength model. Krill density ranged over several orders of magnitude (0–10 000 g m−2) and its distribution was highly skewed with many zero observations. Within each survey, the mean krill density was significantly correlated with the top 7% of the maximum krill densities observed. Hence, only the densest krill swarms detected in any one year drove the mean krill density estimates for the WCB in that year. WCB krill density (µ, mean density for the area) showed several years (1997/1998, 2001–2003, 2005–2007) of high values (µ > 30 g m−2) interspersed with years (1999/2000, 2004, 2009/2010) of low density (µ < 30 g m−2). This pattern showed three different periods, with fluctuations every 4–5 years. Cross correlation analyses of variability in krill density with current and lagged indices of ocean (sea surface temperature, SST and El Niño/Southern Oscillation) and atmospheric variability (Southern Annular Mode) found the highest correlation between krill density and winter SST (August SST) from the preceding year. A quadratic regression (r2 = 0.42, p < 0.05) provides a potentially valuable index for forecasting change in this ecosystem.

scholarly journals On the echo interference in sound backscattering by densely aggregated targets

2005 ◽  
Vol 62 (4) ◽  
pp. 771-778 ◽  
Author(s):  
Natalia Gorska ◽  
Dezhang Chu
Keyword(s):  
Analytical Approach ◽  
Three Dimensional ◽  
Biomass Estimation ◽  
Acoustic Backscattering ◽  
Biological Targets ◽  
Wide Range ◽  
Backscattering Strength ◽  
Sampling Volume ◽  
Directivity Patterns ◽  
Numerical Approaches

Abstract It is important to understand the volume backscattering by dense aggregations of a variety of scattering objects such as bubbles or biological targets. This paper addresses the interference of the echoes from randomly distributed targets. The main motivation of the paper is to understand the conditions under which the echo interference may affect the accuracy of the abundance and/or the biomass estimation in fisheries and zooplankton acoustics significantly. Our approach consists of two parts. The first includes an analytical approach, which describes explicitly the dependence of the echo interference on the pulse shape of the transmitted signals. Because of the limitations of the analytical approach, numerical computations based on Monte Carlo simulations of acoustic backscattering by three-dimensional target distribution were performed as a second approach. The impacts of the echo interference were studied numerically over a wide range of frequencies, for different pulse shapes and directivity patterns of the acoustic systems, and for various spatial distributions of the targets (abundance), as well as the corresponding target strengths. Using analytical and numerical approaches it was demonstrated that for targets that are uniformly distributed in space, the influence of echo interference on the observed volume-backscattering strength is strongly controlled by three main parameters. These are the number of targets in the sampling volume, the product of sound frequency and pulse duration, and the degree of tapering of the applied pulses. A numerical examples of the abundance estimation of marine organisms are presented.

scholarly journals Improved parameterization of the SDWBA for estimating krill target strength

2006 ◽  
Vol 63 (5) ◽  
pp. 928-935 ◽  
Author(s):  
Stéphane G. Conti ◽  
David A. Demer
Keyword(s):  
Antarctic Krill ◽  
Euphausia Superba ◽  
Orientation Distribution ◽  
Target Strength ◽  
Model Parameters ◽  
Distorted Wave ◽  
Acoustic Frequency ◽  
General Application ◽  
Broad Bandwidth ◽  
Phase Variability

Abstract Recently, a Stochastic Distorted Wave Born Approximation (SDWBA) model was proposed to improve target strength (TS) estimates for Antarctic krill, Euphausia superba. The krill shape is modelled by a collection of cylinders, and total sound scatter is estimated by semi-coherent summation of scatter from each element. The SDWBA model was evaluated with a generic krill shape comprising 14 cylinders and a phase variability of , and predictions were validated with empirical TS and total TS data at 120 kHz, and over a broad bandwidth, respectively. For general application, parameterization of the SDWBA model is improved to account explicitly for dependence among four of the model parameters: standard length of krill, number of cylinders used to describe its shape, amplitude of inter-element phase variability, and acoustic frequency. The model improvements are demonstrated, and the uncertainty in orientation distribution of krill beneath survey vessels and its ramifications on krill biomass estimates are highlighted.

scholarly journals The target strength of Antarctic krill (Euphausia superba) measured by the split-beam method in a small tank at 70 kHz

2006 ◽  
Vol 63 (1) ◽  
pp. 36-45 ◽  
Author(s):  
Kazuo Amakasu ◽  
Masahiko Furusawa
Keyword(s):  
Antarctic Krill ◽  
Incident Angle ◽  
Euphausia Superba ◽  
Side Lobe ◽  
Target Strength ◽  
Model Predictions ◽  
And Training ◽  
Small Tank ◽  
Antarctic Krill Euphausia Superba ◽  
Good Agreement

Abstract The target strengths (TS) of Antarctic krill (Euphausia superba) were measured at 70 kHz aboard the research and training vessel “Umitaka-maru” of Tokyo University of Marine Science and Technology in February 2003 during a Southern Ocean survey. The systematic variations of TS vs. the incident angle of the ensonified wave, henceforth called TS-patterns, were successfully measured for 12 live Antarctic krill. These measurements were compared with the theoretical TS-patterns predicted by the Distorted-Wave Born Approximation-based deformed-cylinder model (DWBA model). While there was good agreement near the main lobe, the measurements were higher than the model predictions in the side-lobe regions; this is consistent with the observations of others. Several possible causes of this discrepancy such as the bending of abdomen and scattering from pleopods were examined, but no single factor was identified as the cause. Rather, it is likely that the discrepancy is a result of a combination of several factors.

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