scholarly journals Further analysis of target strength measurements of Antarctic krill at 38 and 120 kHz: Comparison with deformed cylinder model and inference of orientation distribution

1993 ◽  
Vol 93 (5) ◽  
pp. 2985-2988 ◽  
Author(s):  
Dezhang Chu ◽  
Kenneth G. Foote ◽  
Timothy K. Stanton
Keyword(s):  
Antarctic Krill ◽  
Orientation Distribution ◽  
Target Strength ◽  
Cylinder Model

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 Modelling the acoustic effect of swimbladder compression in herring

2003 ◽  
Vol 60 (3) ◽  
pp. 548-554 ◽  
Author(s):  
Natalia Gorska ◽  
Egil Ona
Keyword(s):  
Orientation Distribution ◽  
Target Strength ◽  
Acoustic Effect ◽  
Acoustic Backscattering ◽  
Cylinder Model ◽  
Acoustic Surveys ◽  
The Mean ◽  
Theoretical Results

Abstract Obtaining accurate data on fish target strength (TS) is important when determining the quality of the results from acoustic surveys. However, this requires an improved understanding of both behavioural and environmental influences on the acoustic backscattering by fish. It is well known that the increased pressure with depth compresses the swimbladder of herring, and it has been confirmed by in situ measurements that the TS of adult herring (30–34 cm) is 3–5 dB weaker at 300 m than that of fish close to the surface. Understanding exactly how swimbladder compression may influence herring TS is, therefore, of great interest, and is the main motivation behind this study. Taking account of swimbladder volume changes with depth, we obtained analytical solutions using the Modal-Based, Deformed-Cylinder Model (MB-DCM). The mean-backscattering cross-section is then computed with selected orientation patterns, length distributions, and contrast parameters. The depth-dependence of TS at different acoustic frequencies has been studied. We conducted a sensitivity analysis to show how TS is dependent on the contraction rates of the bladder dimensions and on the fish-orientation distribution. Our theoretical results are compared with TS measured at 38 kHz.

scholarly journals Improving the Accuracy of Krill Target Strength Using a Shape Catalog

2021 ◽  
Vol 8 ◽  
Author(s):  
Fiona Bairstow ◽  
Sven Gastauer ◽  
Luke Finley ◽  
Tom Edwards ◽  
C. Tom A. Brown ◽  
...  
Keyword(s):  
Antarctic Krill ◽  
Length Distribution ◽  
Practical Method ◽  
Orientation Distribution ◽  
Mie Scattering ◽  
Target Strength ◽  
Limiting Factor ◽  
Backscattering Cross Section ◽  
Orientation Distributions ◽  
Strength Models

Antarctic krill are subject to precautionary catch limits, based on biomass estimates, to ensure human activities do not adversely impact their important ecological role. Accurate target strength models of individual krill underpin biomass estimates. These models are scaled using measured and estimated distributions of length and orientation. However, while the length distribution of a krill swarm is accessible from net samples, there is currently limited consensus on the method for estimating krill orientation distribution. This leads to a limiting factor in biomass calculations. In this work, we consider geometric shape as a variable in target strength calculations and describe a practical method for generating a catalog of krill shapes. A catalog of shapes produces a more variable target strength response than an equivalent population of a scaled generic shape. Furthermore, using a shape catalog has the greatest impact on backscattering cross-section (linearized target strength) where the dominant scattering mechanism is mie scattering, irrespective of orientation distribution weighting. We suggest that shape distributions should be used in addition to length and orientation distributions to improve the accuracy of krill biomass estimates.

Estimation of Fish Length from Acoustic Target Strengths

1987 ◽  
Vol 44 (10) ◽  
pp. 1782-1785 ◽  
Author(s):  
U. Buerkle
Keyword(s):  
Gadus Morhua ◽  
Statistical Significance ◽  
Orientation Distribution ◽  
Target Strength ◽  
Fish Length ◽  
Fish Sample ◽  
Length Frequency ◽  
Frequency Structure ◽  
Acoustic Target

I show that fish length distributions calculated from acoustic target strengths of cod (Gadus morhua) are in error when target strength–length relationships are used that do not account for the orientation distribution of the fish. The magnitude and statistical significance of the errors vary with the length frequency structure of the fish sample and are thought to be large enough to question the value of acoustically determined lengths in fish surveys if corrections for fish orientation are not applied.

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 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.

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