Lateral target strength of Antarctic krill

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.

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

ICES Journal of Marine Science ◽

10.1093/icesjms/fsp040 ◽

2009 ◽

Vol 66 (6) ◽

pp. 1245-1251 ◽

Cited By ~ 6

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.

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Sexual dimorphism in body shape of Antarctic krill (Euphausia superba) and its influence on target strength

Polar Science ◽

10.1016/j.polar.2011.04.005 ◽

2011 ◽

Vol 5 (2) ◽

pp. 179-186 ◽

Cited By ~ 4

Author(s):

Kazuo Amakasu ◽

Atsushi Ono ◽

Masato Moteki ◽

Takashi Ishimaru

Keyword(s):

Sexual Dimorphism ◽

Body Shape ◽

Antarctic Krill ◽

Euphausia Superba ◽

Target Strength ◽

Antarctic Krill Euphausia Superba

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Improved parametrization of Antarctic krill target strength models

The Journal of the Acoustical Society of America ◽

10.1121/1.2141229 ◽

2006 ◽

Vol 119 (1) ◽

pp. 232-242 ◽

Cited By ~ 51

Author(s):

Gareth L. Lawson ◽

Peter H. Wiebe ◽

Carin J. Ashjian ◽

Dezhang Chu ◽

Timothy K. Stanton

Keyword(s):

Antarctic Krill ◽

Target Strength ◽

Strength Models

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

ICES Journal of Marine Science ◽

10.1016/s1054-3139(03)00063-8 ◽

2003 ◽

Vol 60 (3) ◽

pp. 625-635 ◽

Cited By ~ 35

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.

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Improved parameterization of the SDWBA for estimating krill target strength

ICES Journal of Marine Science ◽

10.1016/j.icesjms.2006.02.007 ◽

2006 ◽

Vol 63 (5) ◽

pp. 928-935 ◽

Cited By ~ 40

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.

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

ICES Journal of Marine Science ◽

10.1016/j.icesjms.2005.07.012 ◽

2006 ◽

Vol 63 (1) ◽

pp. 36-45 ◽

Cited By ~ 16

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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