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Review and recommendations for the modeling of acoustic scattering by fluid-like elongated zooplankton: Euphausiids and copepods
Abstract
Acoustic echo sounders are commonly used to survey zooplankton. An essential element in the methods is the acoustic scattering model, which relates acoustic echo data to meaningful biological parameters such as size and numerical density. Because of the importance of scattering models, there has been much development of models of increasing sophistication. With the increase in sophistication is an associated improvement in accuracy, but possibly at the cost of increased effort in implementing the model. Thus the practical question is which model provides sufficient accuracy for the scientific problem of interest. This paper presents a modelling study using a wide range of models, ranging from simple to complex representation of the animals, a synthesis of previously published laboratory scattering data from a variety of sources, and laboratory data presented for the first time. The focus is on fluid-like zooplankton (i.e., animals that do not support shear waves) with examples specific to euphausiids, shrimp, and copepods. The simplest model is the sphere with homogeneous material properties while the most complex is a high-resolution digitized form of each zooplankton taking into account surficial roughness and inhomogeneities of the material properties. The results shove that there are conditions under which very simple, easy-to-use models can be used with reasonable accuracy while there are other conditions where the more complex models must be used.
... If the elongated suspended particles are oriented vertically or inclined, the amplitude of A 3 will significantly differ from the amplitudes of A 1 and A 2 . This was shown for copepods based on both models of acoustic scattering at a frequency of 2 MHz (Stanton and Chu, 2000;Roberts and Jaffe, 2007) and laboratory experiments (Roberts and Jaffe, 2008). ...
... The combination of horizontal and tilted beam signals allows, on the one hand, the patches of particles to be eliminated and the background scattering level of the echogram to be equalized and, on the other hand, the preferred orientation of mesozooplankton species migrating through the oxycline to be determined. Earlier, Stanton and Chu (2000) reproduced the influence of the orientation of a 3 mm calanoid copepod (modeled as a high-resolution approximation of an animal profile) on the acoustic target strength at 2 MHz with respect to an incident sonar beam. The reduction was found to be 5 %-15 % when copepod orientation was shifted from 0 • (broadside incidence) to 30-60 • . ...
... To distinguish the SSLs against the background patterns of vertical flow of settling particles and to study the orientation of zooplankton species, we propose a simple method for the processing of ultrasound sensing data at three angles. This acoustic three-beam geometry provides a partial pragmatic solution for the quest towards the multiple-angle scat-ter measurements suggested by models (Stanton and Chu, 2000;Roberts and Jaffe, 2007) and laboratory experiments (Roberts and Jaffe, 2008). Since the late 1990s, researchers' efforts have been focused on creating multichannel instruments to measure acoustic backscatter (volume scattering strength) at several frequencies, which contain information about the size composition of the scatterers, since different frequencies bounce off objects of different sizes (Wiebe Smeti et al., 2015). ...
At the northeastern Black Sea research site, observations from 2010–2020 allowed us to study the dynamics and evolution of the vertical distribution of mesozooplankton in oxygen-deficient conditions via analysis of sound-scattering layers associated with dominant zooplankton aggregations. The data were obtained with profiler mooring and zooplankton net sampling. The profiler was equipped with an acoustic Doppler current meter, a conductivity–temperature–depth probe, and fast sensors for the concentration of dissolved oxygen [O2]. The acoustic instrument conducted ultrasound (2 MHz) backscatter measurements at three angles while being carried by the profiler through the oxic zone. For the lower part of the oxycline and the hypoxic zone, the normalized data of three acoustic beams (directional acoustic backscatter ratios, R) indicated sound-scattering mesozooplankton aggregations, which were defined by zooplankton taxonomic and quantitative characteristics based on stratified net sampling at the mooring site. The time series of ∼ 14 000 R profiles as a function of [O2] at depths where [O2] < 200 µm were analyzed to determine month-to-month variations of the sound-scattering layers. From spring to early autumn, there were two sound-scattering maxima corresponding to (1) daytime aggregations, mainly formed by diel-vertical-migrating copepods Calanus euxinus and Pseudocalanus elongatus and chaetognaths Parasagitta setosa, usually at [O2] = 15–100 µm, and (2) a persistent monospecific layer of the diapausing fifth copepodite stages of C. euxinus in the suboxic zone at 3 µm < [O2] < 10 µm. From late autumn to early winter, no persistent deep sound-scattering layer was observed. At the end of winter, the acoustic backscatter was basically uniform in the lower part of the oxycline and the hypoxic zone. The assessment of the seasonal variability of the sound-scattering mesozooplankton layers is important for understanding biogeochemical processes in oxygen-deficient waters.
... To distinguish the SSLs against the background patterns of vertical flow of settling particles and to study the orientation of zooplankton spices, we propose a simple method 70 for the processing of ultrasound sensing data at three angles. This acoustic 3-beam geometry may therefore substantially reduce the development and deployment costs associated with underwater acoustic scattering apparatuses, thereby providing a partial pragmatic solution for the quest towards the multiple-angle scatter measurements suggested by models (Stanton, Chu, 2000;Roberts and Jaffe, 2007) and laboratory experiments (Roberts and Jaffe, 2008). It should be noted that since the late 1990s, researchers' efforts have been focused on creating multichannel instruments to measure acoustic backscatter 75 (volume scattering strength) at several frequencies, such as the Tracor Acoustic Profiling System operating at the six frequencies of 265, 420, 710, 1100, 1850, and 3000 kHz (Smeti et al., 2015). ...
... If the elongated suspended particles are oriented vertically or inclined, the amplitude of A 3 will significantly differ from the amplitudes of A 1 and A 2 . This was shown for copepods, based on both models of acoustic scattering at a frequency of 2 MHz (Stanton and Chu, 2000;Roberts and Jaffe, 2007) and laboratory experiments (Roberts and Jaffe, 2008). ...
... The combination of horizontal and tilted beam signals allows, on the one hand, eliminating the time variability associated with the patches of particles and equalizing the background scattering level of the echogram and, on the other hand, determining the preferred orientation of mesoplankton species migrating through the oxycline. Earlier, Stanton and Chu (2000) reproduced the influence of the orientation of a 3-mm calanoid copepod (modeled as a highresolution approximation of an animal profile) on the acoustic target strength at 2 MHz with respect to an incident sonar 335 beam. The reduction was found to be 5-15% when copepod orientation was shifted from 0° (broadside incidence) to 30-60°. ...
To investigate the annual cycle of sound-scattering layers in the Black Sea, a moored profiler equipped with an acoustic Doppler current meter, a conductivity-temperature-depth probe, and fast sensors for dissolved oxygen [O2] was employed. Approximately 13,350 multiparameter profiles from the near-surface layer down to the near-bottom layer were obtained at intervals of 1-2 h from 2013-2020. The acoustic system allowed for observations of ultrasound backscattering at 3 angles at 2 MHz frequency. Combinations of the volume strength data of the 3 acoustic beams (directional acoustic 10 backscatter ratios, R) were found to be a useful tool for visualizing acoustic backscatter patterns associated with mesoplankton in the oxycline and hypoxic zone. The time series of R as a function of [O2] at depths where [O2] < 200 μM were analyzed to determine the annual cycle of sound-scattering mesoplankton aggregations. It was shown that from spring to early autumn, there are two sound-scattering maxima corresponding to the daytime aggregations of diel-vertical-migrating specimens usually at [O2] = 20-60 μM and the persistent layer of diapausing specimens at [O2] < 10 μM. During the rest of 15 autumn until early winter, there is usually no persistent deep sound-scattering layer, while the maximum corresponding to the daytime mesoplankton aggregations shifted deeper to [O2] = 10-30 μM. During the rest of winter, the acoustic backscatter is basically uniform throughout the water column. The R graphs also indicate that the mesoplankton specimens tended to be oriented vertically in the lower part of the oxycline and hypoxic zone.
... For smaller organisms, direct observation of TS is generally not possible because it is difficult to resolve them individually. Thus, TS is theoretically calculated using physics-based scattering models, often in conjunction with laboratory-based measurements for verification (Greenlaw 1977;Stanton and Chu 2000;Chu and Wiebe 2005;Lawson et al. 2006;Lavery et al. 2007;Warren and Wiebe 2008). Such models allow for the prediction of TS at a range of frequencies, which is often of substantial value in studies of zooplankton employing multiple frequencies or broadband (Pieper and Holliday 1984;Napp et al. 1993;Stanton et al. 1996;Stanton and Chu 2000;Holliday et al. 2003;Lavery et al. 2007). ...
... Thus, TS is theoretically calculated using physics-based scattering models, often in conjunction with laboratory-based measurements for verification (Greenlaw 1977;Stanton and Chu 2000;Chu and Wiebe 2005;Lawson et al. 2006;Lavery et al. 2007;Warren and Wiebe 2008). Such models allow for the prediction of TS at a range of frequencies, which is often of substantial value in studies of zooplankton employing multiple frequencies or broadband (Pieper and Holliday 1984;Napp et al. 1993;Stanton et al. 1996;Stanton and Chu 2000;Holliday et al. 2003;Lavery et al. 2007). ...
... Distorted-wave Born approximation (DWBA)-based models have emerged as the most common approach for TS estimation of weakly scattering zooplankton such as copepods and krill (Stanton and Chu 2000;Demer and Conti 2005;Calise and Skaret 2011). Predictions of these models have been experimentally verified in many instances with laboratory and field measurements Lawson et al. 2006). ...
Estimation of abundance or biomass, using acoustic techniques requires knowledge of the frequency dependent acoustic backscatter characteristics, or target strength, of organisms. Target strength of zooplankton is typically estimated from physics‐based models that involve multiple parameters, notably including the acoustic material properties (i.e., the contrasts in density and sound speed between the animal and surrounding seawater). In this work, variability in the acoustic material properties of two zooplankton species in the Gulf of Maine, the copepod (Calanus finmarchicus) and krill (Meganyctiphanes norvegica), was investigated relative to changing season as well as, for the copepod, temperature and depth. Increases in the density and sound speed contrasts of these species from fall to spring were observed. Target strength predictions based on these measurements varied between fall and spring by 2‐3 dB in krill. Measurements were also conducted on C. finmarchicus lipid extract at changing temperature and pressure. The density contrast of the extract varied negatively with temperature, while the sound speed contrast changed by more than 10 % over the temperature and pressure ranges that the organism expected to occupy. C. finmarchicus target strength predictions showed that the combined effect of temperature and pressure can be significant (more than 10 dB) due to the varying response of lipids. The large vertical migration ranges and lipid accumulation characteristics of these species (e.g., the diapause behaviour of Calanus copepods) suggest that it is necessary for seasonal and environmental variability in material properties to be taken into account to achieve reliable measurements.
... If the impedance and shape are known, inversions of scattering models for individual scatterers can be used to determine the remaining model input: size. Jech et al. (2015) and Stanton and Chu (2000) review a variety of acoustic scattering models of increasing complexity, both computationally and in terms of the target shape. Weber et al. (2012) used a simplified model of acoustic scattering that assumes droplets were spherical to determine oil flux during the DWH spill. ...
... The experimental results are compared to broadband backscattering modeled by three commonly used models: the modal solution derived by Anderson (1950), the DWBA (Stanton et al., 1998), and the BEM (Francis and Foote, 2003;Okumura et al., 2003). These three models have been used to predict the scattering from a variety of targets, for example, the Anderson (1950) model has been used to predict scattering from euphausiids and oil droplets (Kristensen and Dalen, 1986;Weber et al., 2012), the DWBA has been used to predict the scattering from a variety of biological targets, including copepods, euphausiids, krill, and squid Jones et al., 2009;Stanton and Chu, 2000), and the BEM has been used to model backscatter from bubbles, fish swimbladders, and swimbladder like shapes (see, e.g., Francis and Foote, 2003;Okumura et al., 2003). The Anderson model assumes that targets have spherical symmetry, while the DWBA can be used for more complex shapes; however, it assumes that targets are weakly scattering. ...
... For the DWBA, scattering is calculated with the magnitude of the incident acoustic wavenumber determined by the sound speed of the scatterer (hence, the term distorted). The DWBA is valid over all ranges of angles of incidence (Stanton and Chu, 2000;Stanton et al., 1998) when differences between the properties of the target and the surrounding medium are small. The general form of the Born approximation is a volume integral derived by Morse and Ingard (1968), ...
Improved in situ quantification of oil in the marine environment is critical for informing models of fate and transport and evaluating the resiliency of marine communities to oil spills. Broadband acoustic backscatter has been used to quantify a variety of targets in the water column; from fish and planktonic organisms to gas bubbles and oceanic microstructure, and shows promise for use in quantifying oil droplets. Quantifying water column targets with broadband acoustic backscatter relies on accurate models of a target's frequency dependent target strength (TS), a function of the target's acoustic impedance, shape, and size. Previous acoustic quantification of oil droplets has assumed that droplets were spheres. In this study, broadband (100.5–422 kHz) acoustic backscatter from individual oil droplets was measured, and the frequency dependent TS compared to a model of acoustic scattering from fluid spheres and two models for more complex shapes. Droplets of three different crude oils, two medium oils, and one heavy oil were quantified and all droplets were oblate spheroids. The impact of the deviation from sphericity on the accuracy of each model was determined. If an inversion of the model for spherical droplets was used to estimate flux from acoustic observations, errors in the predicted volume of a droplet were between 30% and 50%. The heavy oil also showed deviations in predicted volume of 20%–40% when using the two models for more complex shapes.
... The first models focused on marine organisms were based on canonical shapes [7]. Then, different schemes allows to deal with complex bodies in an approximated way, see Ref. [6,8] and references therein. Further, use of the Born approximation was extended to bodies whose geometry can be described as a collection of non-circular rings centred on a smooth curve [9]. ...
... Absolute value |f ∞ | for the backscattering by a spheroid under the DWBA according to the exact formula(8) and two executions of TetraScatt with the indicated meshes. Top: broadside incidence. ...
Modelling the acoustic scattering response due to penetrable objects of arbitrary shapes, such as those of many marine organisms, can be computationally intensive, often requiring high-performance computing equipment when considering a completely general situation. However, when the physical properties (sound speed and density) of the scatterer are similar to those of the surrounding medium, the Born approximation provides a computationally efficient way to calculate it. For simple geometrical shapes like spheres and spheroids, the acoustic scattering in the far-field evaluated through the Born approximation recipe results in a formula which has been historically employed to predict the response of weakly scattering organisms, such as zooplankton. Further, the Born approximation has been extended to bodies whose geometry can be described as a collection of non-circular rings centred on a smooth curve. In this work, we have developed a numerical approach to calculate the far-field backscattering by arbitrary 3D objects under the Born approximation. The object's geometry is represented by a volumetric mesh composed of tetrahedrons, and the computation is efficiently performed through analytical 3D integration, yielding a solution expressed in terms of elementary functions. The method's correctness has been successfully validated against benchmark solutions. Additionally, we present acoustic scattering results for species with complex shapes. To enable other researchers to use and validate the method, a computational package named tetrascatt was developed in the R programming language and published in the CRAN (Comprehensive R Archive Network).
... Potential candidates of each echo-class were investigated by classifying theoretical organisms, whose TS(f) was calculated using previously published and validated theoretical scattering models. The scattering models used to investigate the biological significance of each echo-class defined from Section 2.6 were the following: Distorted Wave Born Approximation (DWBA) with parameters for copepods and euphausiids [34,35]; randomly oriented fluid bent cylinder with parameters for shrimp and salp [36]; high-pass dense fluid sphere with parameters for gastropods [36]; hybrid model with parameters for pneumatophores of siphonophores and small gaseous swimbladdered fish [37]; model for gas bubble alone [38]. Each scattering model was parametrized using values of speed of sound in the water calculated from temperature and salinity at 25 m, and 200 m depths using the Mackenzie equation, density contrast (for lack of other measures), orientation and the length-to-width ratio of organisms as determined previously by [35][36][37] (Refer to S3 File for parameters used in the scattering models). ...
... Broad categories of scatterers have distinct frequency response curves in the 10-200 kHz frequency band [9]. The relative frequency response has been widely used as a basic indicator for attributing acoustic backscatter energy to the broad categories of scatterers [8,10] over a wide range of animal sizes (or acoustic frequencies) [12,34,37,48]. ...
The impact of a cyclonic (C), an anticyclonic (AC) eddy and transition zone (TZ), which is the area between the two eddies, on acoustic groups representing various mesopelagic organisms, was investigated using a semi-supervised multifrequency classification approach (hereafter, Escore algorithm). The Escore algorithm involved selecting regions of interest (ROIs) within multifrequency (18, 38, 70, and 120 kHz) echograms and classifying into four clusters or echo-classes using Sv differences (Sv18-38, Sv70-38, and Sv120-38). Acoustic densities and diel vertical migration strength varied between the AC, C, and TZ according to the frequency. The vertical stratification of temperature, salinity and fluorescence within the oceanographic structures had varied influences on the vertical structure of each echo-class which represent zooplankton-like organisms, small and large fish with swimbladders, and small and large siphonophores with pneumatophores. The echo-classes within the C were influenced by surface fluorescence, whereas in the AC and TZ, the echo-classes were influenced by deeper fluorescence and strong EKE. Our study provides new insights into the environmental variables within mesoscale and sub-mesoscale features impacting different groups of mesopelagic communities in the Indian Ocean.
... TS variability is another factor that could contribute to the differences between the two methods. Copepods exhibit natural variations in size, orientation, and composition, which affect their acoustic reflectivity [99]. If the modelling assumptions do not adequately account for these variations, the acoustic method can overestimate copepod abundance [100]. ...
... Furthermore, TS models can be sensitive to the acoustic properties of the materials, i.e. the contrasts in sound speed (h) and density (g) between the animal and the surrounding seawater [99]. Parameters used in the model, such as g and h, were taken from the literature, and their real values for the precise species of this study are unknown. ...
The Canary Current Large Marine Ecosystem (CCLME) is one of the most productive Large Marine Ecosystems worldwide. Assessing the abundance, biomass and distribution of zooplankton in the southern part of this system, off the coast of West Africa, remains challenging due to limited sampling efforts and data availability. However, zooplankton is of primary importance for pelagic ecosystem functioning. We applied an inversion method with combined analysis of acoustic and biological data for copepod discrimination using a bi-frequency (38 and 120 kHz) approach. Large copepods with equivalent spherical radii > 0.5 mm were identified using differences in the mean volume backscattering strength (MVBS). Regarding abundance measured by net sampling, copepods strongly dominated the zooplankton community and the large fraction account for 18%. This estimate correlated significantly with MVBS values that were obtained using an inverse algorithm. We confirmed the utility of using 38 kHz for large copepod detection. An epipelagic biomass of large copepod was estimated at 120–850 mg m⁻² in March during upwelling season. It is worth noting that this estimation likely underestimates the true biomass due to inherent uncertainties associated with the measurement method. We recommend future investigations in the interest of using only nighttime data to improve the sampling pattern, particularly on the upper part of the water column (< 10 m) as well as on the shallow part of the continental shelf (< 20 m depth) not covered by fisheries vessel. Nevertheless, such high copepod biomass supports high fish production underlining the key role of copepod in the CCLME. Our results open the way to the analysis of the fluctuation and trend of copepod biomass, along with three decades of fisheries acoustics data available in the region. This helps to determine ecosystem changes, particularly under climate change, and to investigate the role of copepods in the southern CCLME carbon pump at the fine scale.
... We applied model-informed classification to a mixed assemblage of Arctic mesozooplankton that was dominated by fluid-like species, i.e. animals with sound scattering properties similar to water (e.g. euphausiids, copepods, and salps) (Stanton and Chu 2000 ). The objectives were threefold: (i) to design an in situ mesocosm experiment to insonify zooplankton in a near-natural environment with minimal background noise and reverberation; (ii) to evaluate the performance of classifiers trained with scattering models for differentiating weakly backscattering mesozooplankton groups; and (iii) to validate the classifier predictions on a known community of zooplankton. ...
... The most abundant were calanoid copepods, euphausiids, chaetognaths, and pelagic hydrozoans. All these groups are considered fluidlike scatterers with sound speed contrast ( h ) and density con-trast ( g ) of 1 ± 5% (Stanton and Chu 2000 ). Near-unity sound speed and density contrasts imply that the material properties of the scatterers are not significantly different from the surrounding medium (seawater). ...
Classification of zooplankton to species with broadband echosounder data could increase the taxonomic resolution of acoustic surveys and reduce the dependence on net and trawl samples for ‘ground truthing’. Supervised classification with broadband echosounder data is limited by the acquisition of validated data required to train machine learning algorithms (‘classifiers’). We tested the hypothesis that acoustic scattering models could be used to train classifiers for remote classification of zooplankton. Three classifiers were trained with data from scattering models of four Arctic zooplankton groups (copepods, euphausiids, chaetognaths, and hydrozoans). We evaluated classifier predictions against observations of a mixed zooplankton community in a submerged purpose-built mesocosm (12 m3) insonified with broadband transmissions (185–255 kHz). The mesocosm was deployed from a wharf in Ny-Ålesund, Svalbard, during the Arctic polar night in January 2022. We detected 7722 tracked single targets, which were used to evaluate the classifier predictions of measured zooplankton targets. The classifiers could differentiate copepods from the other groups reasonably well, but they could not differentiate euphausiids, chaetognaths, and hydrozoans reliably due to the similarities in their modelled target spectra. We recommend that model-informed classification of zooplankton from broadband acoustic signals be used with caution until a better understanding of in situ target spectra variability is gained.
... This is an advantage in acoustics, since it allows models to include arbitrarily complex scattering models (e.g. modal solutions for spheres and bubbles, distorted-wave Born approximations (DWBAs) including numerical integration, etc., Stanton and Chu, 2000 ;Jech et al., 2015 ). Additionally, because the models are specified cell-bycell, they can be fit in parallel on as many computer cores as desired, whether locally or in the cloud. ...
... We assumed that the TS of a larval fish would be dominated by scattering from its swimbladder, approximated as a damped bubble with equivalent spherical radius (ESR) a = 0.7 mm and damping parameter δ = 0.3, located at 200 m depth, giving it a resonant frequency of 22 kHz (Love, 1978 ). Krill TS was modelled based on a fluid-like deformed cylinder using the DWBA ( McGehee et al., 1998 ;Stanton and Chu, 2000 ). We assumed the same krill shape as McGehee et al. (1998) , but reduced the body length to 25 mm to better match the size of krill encountered during our surveys, scaling down all dimensions proportionally. ...
Identifying sound-scattering organisms is a perennial challenge in fisheries acoustics. Most practitioners classify backscatter based on direct sampling, frequency-difference thresholds, and expert judgement, then echo-integrate at a single frequency. However, this approach struggles with species mixtures, and discards multi-frequency information when integrating. In version methods do not ha v e these limitations, but are not widely used because species identifications are often ambiguous and the algorithms are complicated to implement. We address these shortcomings using a probabilistic, Bayesian in version method. Like other inversion methods, it handles species mixtures, uses all available frequencies, and extends naturally to broadband signals. Unlike previous approaches, it leverages Bayesian priors to rigorously incorporate information from direct sampling and biological knowledge, constraining the inversion and reducing ambiguity in species identification. Because it is probabilistic, a well-specified model should not produce solutions that are both wrong and confident. The model is based on physical scattering processes, so its output is fully interpretable, unlike some machine learning methods. Finally, the approach can be implemented using existing Bayesian libraries and is easily parallelized f or large datasets. We present examples using simulations and field data from the Gulf of Alaska, and discuss possible applications and extensions of the method.
... Since scattering from individual krill is highly non-linear, the conventional linear regression between logarithm of body length and acoustic target strength, typically used for fish, can be inaccurate (Stanton et al., 1994;Demer and Martin, 1995). Instead, models of krill target strength use physical representations of the krill body under a given set of parameters taking into account krill body composition and/or behaviour (Stanton and Chu, 2000). The Distorted Wave Born Approximation acoustic scattering model (DWBA) (Stanton and Chu, 2000;Demer and Conti, 2005) with stochastic enhancements (SDWBA) CCAMLR, 2010b;Calise and Skaret, 2011) has been adopted by CCAMLR for biomass estimation. ...
... Instead, models of krill target strength use physical representations of the krill body under a given set of parameters taking into account krill body composition and/or behaviour (Stanton and Chu, 2000). The Distorted Wave Born Approximation acoustic scattering model (DWBA) (Stanton and Chu, 2000;Demer and Conti, 2005) with stochastic enhancements (SDWBA) CCAMLR, 2010b;Calise and Skaret, 2011) has been adopted by CCAMLR for biomass estimation. Following the CCAMLR protocol, the model is used to estimate target strength based on a given set of parameters defining krill morphology, structure, and behaviour (see Table 3 in Krafft et al., 2021 for parameter settings used). ...
Antarctic krill is a key species in the Southern Ocean and subject to the most extensive fishery in the Antarctic. The Norwegian Institute of Marine Research has conducted acoustic-trawl monitoring of krill off the South Orkneys annually since 2011 in collaboration with the krill fishing industry. Average krill biomass within the 60000 km2 survey area ranged from 1.4 to 7.8 million tonnes in the period 2011–2020, strongly supporting that this is among the regions in the Scotia Sea with consistently highest krill densities. There were no significant (p ≈ 0.18, non-parametric Mann–Kendall test) monotonic trends in estimated krill biomass over the 10 years. The highest krill densities were associated with the shelf edge and submarine canyons on the north side of the South Orkneys. Our comparison with the CCAMLR 9.3% reference exploitation rate suggests that management of the krill fishery in the South Orkneys region is precautionary. The monitoring is run on fishing vessels, so e.g. acoustic frequencies applied could not always be in compliance with the standard CCAMLR methodology. Estimated deviance in krill backscatter when comparing 38 kHz to the standard 120 kHz ranged from −1.1% to 12.8%. Our results show that industry-based surveys are cost-efficient approaches to high-quality monitoring of krill.
... The three vertebrate species were modeled using the KRM model . The squid was modeled using the DWBA for a bent cylinder (Stanton and Chu, 2000;Stanton et al., 1998). The density contrast g and sound speed contrast h for each species are listed in Table III. ...
... The density contrast g and sound speed contrast h are listed in Table III. The broadband backscattering for longfin squid was predicted using a distorted wave Born approximation (DWBA) based prolate spheroid scattering model (Lee et al., 2012;Stanton and Chu, 2000;Stanton et al., 1998). f red bs for squid was estimated using a length-to-width ratio of 9.0:1.2. ...
At the New England shelf break, cold, less saline shelf water collides with warmer saltier slope water to form a distinct oceanographic front. During the Office of Naval Research Sediment Characterization Experiment in 2017, the front was mapped by narrowband (18 and 38 kHz) and broadband (70–280 kHz) shipboard echo sounders. The acoustically determined cross-shelf velocity of the front ranged in amplitude from 0.02 to 0.33 m/s. Acoustic surveys revealed aggregations of scatterers near the foot of the front. Acoustic backscatter in conjunction with Northeast Fisheries Science Center bottom trawl surveys identified longfin squid ( Doryteuthis pealeii) and mackerel ( Scomber scombrus) as the most likely scatterers in the aggregations. A mixed species scattering model was developed and further refined by the use of a matching method used for distribution of the lengths of each species. The mean length of squid and mackerel, respectively, using the matching method was 4.45 ± 1.00 and 20.25 ± 1.25 cm compared with 6.17 ± 2.58 and 22.76 ± 1.50 cm from the trawl data. The estimated total biomass of the aggregation was a factor of 1.64 times larger when using the matching method estimated length distribution compared to the trawl length distribution.
... We determined from Zoocam images that six taxonomic groups likely dominated the scattering of the mesozooplankton in our study site during these missions, namely copepods, chaetognaths, euphausiids, appendicularians, cnidarians, and doliolids/salps . The theoretical scattering properties of these taxonomic groups were approximated through simulations using the distorted wave born approximation (DWBA) (Stanton et al. 1998;Chu and Ye 1999;Stanton and Chu 2000) within the R (R Core Team 2020, v. 4.0.3) package ZooScatR (Gastauer et al. 2019). ...
... As acoustic targets, zooplankton generally are referred to as weak, fluid-like scatterers (Stanton et al. 1998;Stanton and Chu 2000). Our modeling shows that linking taxonomic composition or size classes with acoustic returns using only the two present frequencies is challenging. ...
Diel vertical migration (DVM) is a common behavior among marine organisms to balance the trade‐off between surface feeding opportunities and predation‐related mortality risk. Body size is a master trait that impacts predation risk to both visual and nonvisual predators. Acoustic measurements from the autonomous Zooglider revealed size‐dependent DVM behaviors in the San Diego Trough. Dual frequency (200 and 1000 kHz) backscatter, in conjunction with physical properties of the ambient water and optical imaging of zooplankton, were recorded during 12 Zooglider missions over 2 yr. Acoustic size‐categories were identified based on the theoretical scattering properties of dominant taxonomic groups identified optically by the Zoocam. Acoustic modeling suggests that the measured acoustic backscatter in this region is largely dominated by copepods, with appreciable contributions from other taxa. We found that larger organisms migrated deeper (245–227 m) and faster (> 20 m h−1) compared to smaller organisms (156 m, > 15 m h−1). Larger organisms entered the upper layer of the water column later in the evening (0.2–1.5 h later) and descended into deeper water earlier in the morning (0.4–3.7 h earlier) than smaller‐bodied organisms, consistent with body size‐dependent visual predation risk. The variability in daytime depths occupied by small, intermediate, and large‐bodied backscatterers was related to the depth of the euphotic zone, again consistent with light‐dependent risk of predation.
... Experimental approaches include ex-situ measurements of dead or live fish and in situ measurements of freely swimming live fish in the natural habitat. The TS changes, owing to physical and biological factors such as frequency, wavelength (λ), sound speed contrast (h), and density contrast (g) as well as the body shape, length, swim bladder, depth, and tilt angle of the fish [13][14][15][16]. It is difficult to measure the TS for every change in these factors. ...
... W(g) = 0.0045 × L T (cm) 3.1596 , R 2 = 0.93 (1) measurements of freely swimming live fish in the natural habitat. The TS changes, owing to physical and biological factors such as frequency, wavelength (λ), sound speed contrast (h), and density contrast (g) as well as the body shape, length, swim bladder, depth, and tilt angle of the fish [13][14][15][16]. It is difficult to measure the TS for every change in these factors. ...
Pacific herring (Clupea pallasii Valenciennes, 1847) is a commercially important species that inhabits the coastal waters of the North Pacific from Korea to California, USA. This study analyzed the target strength (TS; dB re 1 m²) of Pacific herring individuals (n = 14, total length (LT) = 21.3–32.3 cm) at 38 and 120 kHz using ex-situ measurements and the Kirchhoff-ray mode (KRM) model. The least-squares regressions of the TS–LT relationship for the ex-situ measurements were TS38kHz = 20 log10(LT) − 70.10 (r = 0.17) and TS120kHz = 20 log10(LT) − 70.59 (r = 0.10). The least-squares regressions for the KRM model were TS38kHz = 20 log10(LT) − 68.39 (r = 0.40) and TS120kHz = 20 log10(LT) − 69.74 (r = 0.49). The b20 value of the KRM model was 1.71 dB higher than that of the ex-situ measurement at 38 kHz but similar at 120 kHz. These results provide basic data to evaluate the distribution and abundance of Pacific herring using fisheries’ acoustic technology.
... Post-processing of acoustic data was done with MATLAB R2015b software, focusing on the krill signal. Copepods did not significantly contribute to the backscatter strength because of the low mean target strength (Stanton and Chu, 2000). By employing target strength dB distribution, we assumed that signals observed in the echograms corresponded to organisms larger than 1 cm, and then, we focused our results on krill taxa. ...
... DVM has been already described for many zooplankton species in marine environments (e.g. Buchholz et al., 1995;Falk-Petersen et al., 2009;Mutlu, 2003;Taki et al., 2005) and explored in complex studies (Cisewski and Strass, 2016;Davis et al., 2005;Stanton and Chu, 2000), with vertical migrations ranging from a few to hundreds of meters (Ringelberg, 2010). In the Argentine continental shelf, krill horizontal distribution, its abundance, its role as a key prey and its biology have been described in several studies (Boltovskoy, 1981;Curtolo et al., 1990;Giménez et al., 2018;Guglielmo and Ianora, 1997;Temperoni et al., 2014;Temperoni et al., 2018). ...
Diel vertical migration (DVM) of krill was studied throughout 36 h at a fixed station (46.05°S, 66.19°W; 98-m depth) located in the center of the San Jorge Gulf, Southern Patagonia area, during February 2014. Using an echosounder system, combined with an autonomous Video Plankton Recorder (Auto-VPR) and Jacknet samplings, we describe the migration pattern, the associated biomass and the macrozooplankton species involved. The net sampling and the Auto-VPR images allowed us to identify the krill species detected in the echosounder signals, which corresponded to Euphausia lucens, Euphausia vallentini and Nematoscelis megalops. The krill community followed a “normal pattern” of DVM, ascending at dusk (~18:30 h) and descending at dawn (~06:30 h), forming a dense layer near the bottom during the day. Krill vertical migration speed was estimated from the echogram data at ~ 1 cm s−1 (1 body length per s for 1-cm-long animal), and the integrated mean biomass was 57.8 g m−2. This study provides a description of temporal and spatial patterns of krill vertical distribution, which should be taken into account when studying the complexity of the SJG ecosystem dynamics and carbon flux.
... TS is a function of the transmitted acoustic frequency, ambient sound speed of seawater, body morphology, behaviour (e.g. orientation in the water column, body curvature, flexure), and material properties (Stanton et al., 1998;Stanton and Chu, 2000;Lawson et al., 2006). Krill TS can be estimated using theoretical scattering models, such as distorted wave Born approximation (DWBA) and other variants (Chu et al., 1993;Stanton et al., 1998, Demer andConti, 2003;Jones et al., 2009), and in situ backscatter measurements (Lawson et al., 2006). ...
... For example, the assumed difference in broadside and perpendicular (relative to the sea surface) in situ orientations of krill can shift theoretical TS as much as 40 dB at frequencies in the geometric scattering region (McGehee et al., 1998;Smith et al., 2013); some models may thus be overly sensitive to changes in orientation thereby underestimating TS (Demer and Conti, 2003). Similarly, small variations in material properties ($2-4%) can yield substantial changes in theoretical TS at certain frequencies (e.g. up to 20 dB, Stanton and Chu, 2000). Consequently, the uncertainty in different parameter estimates and how each propagates to differences in TS model outputs needs to be better understood. ...
Target strength model inputs including morphometry, material properties, lipid composition, and in situ orientations were measured for sub-Arctic krill (Euphausia pacifica, Thysanoessa spinifera, T. inermis, and T. raschii) in the eastern Bering Sea (EBS, 2016) and Gulf of Alaska (GOA, 2017). Inter-species and -regional animal lengths were significantly different (F1,680 = 114.10, p < 0.01), while animal shape was consistent for all species measured. The polar lipid phosphatidycholine was the dominant lipid, comprising 86 ± 16% (mean ± SD) and 56 ± 22% of total lipid mass in GOA and EBS krill, respectively. Krill density contrasts varied by species and region rather than with morphometry, lipid composition, or local chla fluorescence. Mean in situ krill orientation was 1 ± 31°, with 25% of observed krill within ±5° of broadside incidence. Modelled target strength sensitivity was frequency independent for variations in material properties but was primarily sensitive to morphometry and orientation at lower (38 kHz) and higher (200 kHz) frequencies, respectively. Measured variability in material properties corresponded to an order of magnitude difference in acoustic estimates of biomass at 120 kHz. These results provide important inputs and constraints for acoustic scattering models of ecologically important sub-Arctic krill species.
... 38 and 120 kHz) is required to better separate fish assemblages backscatter from the acoustic signal of the zooplankton in the pelagic zone (Trenkel & Berger, 2013). Based on the physical properties of the zooplankton, a multifrequency approach will allow the identification of plankton specific groups (Ballón et al., 2011;Simmonds & MacLennan, 2005;Stanton & Chu, 2000), such as fluid-like organisms (e.g. euphausiids, copepods, salps, siphonophores without gas bladder and other large crustacean and decapod larvae; Stanton et al., 1996; T A B L E 3 Date and impact zone of eddies interacting with JFA Stanton & Chu, 2000) and gas-bearing organisms (e.g. ...
... Based on the physical properties of the zooplankton, a multifrequency approach will allow the identification of plankton specific groups (Ballón et al., 2011;Simmonds & MacLennan, 2005;Stanton & Chu, 2000), such as fluid-like organisms (e.g. euphausiids, copepods, salps, siphonophores without gas bladder and other large crustacean and decapod larvae; Stanton et al., 1996; T A B L E 3 Date and impact zone of eddies interacting with JFA Stanton & Chu, 2000) and gas-bearing organisms (e.g. fish, gelatinous and siphonophores larvae with a gas bladder; Ballón et al., 2011). ...
• The Juan Fernandez Ridge, a vulnerable marine ecosystem located far off the coast of central Chile and formed by several seamounts, guyots and three islands (Robinson Crusoe, Santa Clara and Alejandro Selkirk), has recently been declared a Coastal Marine Protected Area of Multiple Uses with several National Parks embedded in it.
• Recent studies have highlighted the influence of remote and local oceanographic structures on the hydrographic dynamics of this ridge. However, there is still a gap in understanding how they affect the structure and dynamics of the surrounding insular planktonic communities.
• A hydroacoustic and oceanographic survey was conducted during the austral spring (October 2016), including hydrographic and zooplankton sampling around Robinson Crusoe Island. Oceanographic features were identified and tracked using satellite data (chlorophyll‐a (Chl‐a) and sea surface temperature) and modelling results.
• Two events of Chl‐a increase relative to a threshold (>0.45 mg m⁻³) were forced by different physical processes, both affecting the western side of Robinson Crusoe Island. In event A during the cruise period, Chl‐a subsurface maxima were associated with the arrival of a coastal meander originating on the continental shelf off Chile (remote process); the zooplankton was dominated by copepods and salps, with an evident coastal–oceanic gradient. In event B, Chl‐a maxima were linked to a local upwelling forced by the intensification of a localized SSW wind. No influence of remote eddies or local Von Kármán vortices on Chl‐a distribution was observed.
• These findings highlight the influence of remote and local physical processes on the structure of planktonic communities around Robinson Crusoe Island. Understanding the variability of these mechanisms and their effects at the base of the pelagic food web is critical in adopting an ecosystem‐level approach.
... In the TS-probe data, copepods could only be isolated when layers could be identified. Otherwise, their contribution to the KRIAM category is expected to be small since TS for copepods is reported to be around −100 dB at 200 kHz for 30 mm copepods (Stanton and Chu, 2000). ...
... Other stations appear to be relatively stable over the water column with consistent with respect to the difference or ratio between the frequencies. The contribution of copepods for this profile cannot be ruled out but according to Stanton and Chu (2000), individual copepods are expected to be 100 times weaker (TS between −100 and −110 dB re 1 m 2 ), and their contribution to the scattering in this interpreted profile is not expected to be large. Automatization of acoustic classification based on frequency response has increased potential for high-resolution data like those coming from the TS-probe. ...
The range limitation (>200 m) for high-frequency echosounders does not allow for complete multifrequency studies of the mesopelagic layers from vessel-mounted echosounders. The layers of mesopelagic fish and zooplankton in the Arctic region north of Svalbard (Spitsbergen) were studied using vessel-mounted echosounders and a profiling acoustic probe, using 38, 120, 200 and 333 kHz. Volume density estimates of mesopelagic fish have shown to be marginally higher with the probing system in relation with measured from the vessel-mounted echosounders at 38 kHz. This shows that the swimbladder resonance phenomenon is not occurring in low density layers with limited vertical migration. The use of the profiling probe allowed densities to be calculated with an in situ measured target strength (TS). In depths >200 m where high-frequency hull-mounted transducers cannot effectively reach, the profiling system measured a mixture of krill and amphipods down to 600 m. Vertical profiles of measured target categories, from vessel transducers and from the probing system are compared in relation to the biological sampling conducted during the survey. Profiling acoustics are shown to be a valuable tool to address some limitations in the current surveying methods for studying mesopelagic layers beyond the reach for high-frequency vessel-mounted systems.
... This correlation indicates that higher zooplankton populations correspond with increased backscatter readings. Volume backscattering is affected by factors such as the abundance and biomass of scatterers, their taxonomic composition, and variations in acoustic properties [74]. Moreover, [26] established a direct correlation between the logarithm of zooplankton density and acoustic volume backscattering strength. ...
The abundance distribution pattern of zooplankton associated with the pre-upwelling and late-upwelling phase is assessed for the eastern Arabian Sea (EAS) summer system, using vessel-mounted moving Acoustic Doppler current profiler (ADCP) and the in situ zooplankton samples collected using plankton nets. The distribution pattern of zooplankton is observed to be regulated by physical factors such as coastal upwelling, circulation patterns, mesoscale eddies, regional stratification, the presence of subsurface chlorophyll-a maximum, etc. during different phases of the upwelling cycle. The volume backscattering strength, a proximate factor for the zooplankton biomass, is computed after deriving the appropriate sound absorption coefficient, slant range, and backscatter noise. The linear relation derived by enumerating the backscatter-to-zooplankton biomass relationship was stronger during the pre-upwelling phase (r = 0.58) but weaker during the late-upwelling phase (r = 0.25). The findings underscore the potential of ADCP backscatter as a reliable indicator of zooplankton biomass within the mixed layer depth of the EAS, especially in the stable, calm, early summer season. The derived equations for estimating biomass are log(B) = 5.39 + 0.05 Sv for pre-upwelling and log(B) = 3.10 + 0.02 Sv for late-upwelling. The reduced correlation later suggests that environmental changes, such as zooplankton size and composition shifts, may affect ADCP’s detection threshold, necessitating careful interpretation. The study shows fish larvae act as dominant scatterers due to their gas-bearing properties, reliably indicating proxies for zooplankton abundance across both upwelling phases. Fluid-like and elastic-shelled scatterers vary between phases, reflecting shifts in zooplankton composition and their impact on acoustic backscatter. The analysis of ADCP backscatter data tracks diel vertical migration (DVM) of zooplankton with significant concentrations at depths of up to approximately 80 m during night-time. This study identifies distinct vertical migration velocities with zooplankton ascending in the range of 7.2 cm/s during dusk and descending at 7.7 cm/s during dawn.
... Acoustic backscatters of the key copepod species were identified by combining acoustic backscatter characteristics and the confirmed vertical distribution from the net surveys ( Figures 3G-I). To determine the Sv 200-120 kHz window range required to identify acoustic backscatter of the key copepods, the acoustic backscatter characteristics of copepods along lengths (1-10 mm) at two frequencies (120 and 200 kHz) were predicted using the distortedwave Born approximation (DWBA) model (Stanton and Chu, 2000;Demer and Conti, 2005) ( Figure 3G). All of the DWBA model input parameters, except for shape, were referenced from the literature values. ...
Diel vertical migration (DVM) of zooplankton plays a vital role in biological carbon pump and food web interactions. However, there is considerable debate about the DVM of zooplankton in response to environmental changes in the Arctic Ocean. We investigated DVM behavior in the key Arctic copepods Calanus glacialis, Calanus hyperboreus, and Metridia longa following the midnight sun period in the East Siberian continental margin region. The two Calanus species showed non-DVM behaviors, whereas M. longa showed a typical DVM pattern consistent with the solar radiation cycle. Additionally, these species showed different vertical distributions. Calanus glacialis was distributed at depths above 20 m in the warm fresh water, where the highest density gradient was observed. Calanus hyperboreus was distributed at depths between 30 and 55 m in the cold salty water, where a high contribution of micro phytoplankton and the subsurface chlorophyll maximum (SCM) layer were observed. M. longa was found across a broader range of temperature and salinity than both Calanus species, and it was distributed in the upper water column, where the SCM layer was observed at night and at depths between 100 and 135 m in the daytime. These results imply that M. longa can be well adapted to the changing Arctic Ocean environment, where sea ice loss and ocean warming are ongoing, whereas C. hyperboreus can be the most vulnerable to these changes. These findings provide important information for understanding variations in the vertical distributions of key copepod species in the rapidly changing Arctic marine environment.
... We also could only evaluate two classification categories (fish and zooplankton), each of which may have contained other species with similar backscatter properties (McKelvey and Wilson 2006). Both zooplankton and fishes vary widely in their acoustic properties, depending on the presence and form of hard parts (e.g., pteropod shells or fish bones) and gas-filled inclusions (e.g., siphonophore floats or fish swimbladders; Stanton and Chu 2000;Rudstam et al. 2012). It remains very difficult to distinguish species composition based only on acoustics if similar backscatter types co-occur (e.g., non-gas-bearing fish and zooplankton; McKelvey and Wilson 2006;Sato et al. 2015). ...
Humpback whales (Megaptera novaeangliae) use southern British Columbia waters to feed, but the type and quantity of prey in many areas used for feeding is unknown. We conducted active acoustic prey mapping in 55 small grid-surveys in two regions off Vancouver Island. We quantitatively compared fish and zooplankton-dominated biomass in known feeding areas with and without foraging humpback whales, and qualitatively described the prey characteristics of the foraged areas. Surveys of the water column suggest that, on average, humpback whale foraging was associated more with increased zooplankton than fish biomass. Prey characteristics varied between the two regions (∼500 km apart), but there was no significant difference in mean backscatter strength in the actively foraged areas between the two regions. Frequency differencing discriminated between the dominant taxa in the water column, but potential epipelagic prey (<10 m) would have been omitted from analysis. However, average depth at the maximum acoustic prey detections was significantly deeper when whales were present (84 m) versus absent (60 m), suggesting predominantly subsurface foraging opportunities suitable to prey mapping.
... Fluid-like organisms. All the remaining acoustics categories, including amphipods, euphausiids and decapods, gelatinous, jellyfish, squid and fish without swimbladder were modelled using a model based on the Distorted-Wave Born Approximation (DWBA) scattering model averaged over a normal distribution of orientation angles (Stanton et al., 1998;Stanton and Chu, 2000). The fluid-like organisms were simplified as uniformly-bent and tapered cylinders, except for jellyfish approximated as two prolate spheroidal surfaces (Warren and Smith, 2007) and integrated over 300 integration points along the body axis of total length (L). ...
Micronekton organisms are a central component of the trophic organization in the pelagic ecosystem, being prey
to top predators and participating in the export of carbon from the surface to the deep layers. Despite their
importance, the abundance estimates and species distribution of micronekton remain largely uncertain. This
study aimed to compare and assess two sampling methods classically used for the estimation of micronekton
abundance in mesopelagic acoustic scattering layers: scientific echosounder and trawl sampling. Measurements
of 38 and 70 kHz hull-mounted echosounders were examined with biological trawl samples from 8 surveys in the
South-West tropical Pacific. A model of acoustic observation was built from the trawl sampling species
composition and forward scattering models. Predicted and observed acoustic responses are compared to assess
the variability and the difference between the acoustic and trawl sampling methods in various scattering layers,
for day and night periods. The difference between methods decreased with depth and with increasing abundance
of fish with swimbladders caught in trawls. Notably, this difference was found to be minimal in the nocturnal
deep scattering layer (mesopelagic zone, depth>200 m). This study emphasizes potential lower estimates of
organisms’ abundance by trawling and a bias towards mesopelagic fish. Understanding the differences between
methods and their variability within different scattering layers is essential for studying micronekton and
improving the precision of biomass estimates
... However, in absence of a swimbladder, the responsible for the acoustic backscattering of fish is a combination of the backscattering of their organs (bones, skull, flesh and internal tissues) which have complex morphological structures that are not as easy to simulate as a spheroid, and have provided less successful, and sometimes divergent, modelling results Gorska et al., 2005;Nesse et al., 2009). Typical models used to predict acoustic backscattering of bladderless fish species found in bibliography are DWBA (Chu et al., 1993;Stanton and Chu, 2000), KRM (Kloser and Horne, 2003) or FEM . ...
Acoustic target strength (TS) is a key parameter for species identification and stock assessment in fisheries. The TS of fish is influenced by the contrast in acoustic impedance between their tissues and the surrounding water. While the swimbladder is responsible for most of the backscatter in fish with gas-filled swim bladders, the backscatter in bladderless fish is a combination of that of fish tissues. The aim of this paper is to explain the marked differences in acoustic properties reported between skipjack tuna and Atlantic mackerel, two important pelagic fish species that lack a swimbladder. The study measures the acoustic properties (density and sound speed) of their flesh and backbone. These measurements are then used to simulate acoustic backscattering using a numerical model based on the Method of Fundamental Solutions, covering a range of frequencies and fish lengths relevant to fisheries acoustics. The numerical estimation of the TS shows that the differences in the material properties of the tissues predict the reported differences in the reduced target strength of more than 10 dB greater for skipjack than for mackerel at 38 kHz and 120 kHz. The study contributes to the understanding of the complex acoustic field backscattered by bladderless fish species and provides insights into the role of fish tissue material properties in the interpretation of acoustic response differences between species.
... Simulated distributions of TS M were highly sensitive to measured variability in model inputs across individual tethered krill that contributed towards the large spread of observed DTS estimates (Figs. 4 and 5). The largest sources of uncertainty-particularly at geometric scattering frequencies-stem from h animal and q c where TS M models for Alaskan krill are particularly sensitive (Lucca et al., 2021) due to shifts in the frequencies where predicted peaks and null TS are predicted (Stanton and Chu, 2000). Capturing precise h animal and q c measurements using a stereocamera system and using splitbeam transducers would help significantly reduce the uncertainty introduced by these variables. ...
Target strength (TS) is commonly used to convert acoustic backscatter from marine organisms to numerical abundance estimates. Shipboard, tank-based TS measurements were made on four sub-Arctic krill species (Euphausia pacifica, Thysanoessa spinifera, Thysanoessa inermis, and Thysanoessa raschii) from the eastern Bering Sea and Gulf of Alaska at discrete frequencies between 42 and 455 kHz. These measurements were compared to scattering model predictions parameterized with data from the same (when possible) individual krill. Statistically significant differences between modeled and experimental estimates at 42, 45, 120, and 131 kHz exceeded 2 dB on average. Variability in the signal-to-noise ratio, animal length, and measurements from two separate narrowband and broadband transducer pairs (at those frequencies) did not account for these differences. Scattering predictions at 120 and 131 kHz were consistent with the expected transition from Rayleigh-to-geometric scattering where models become increasingly sensitive to orientation and body shape variability. Disagreement between modeled and measured TS may be due to using scattering models developed for, and validated on, larger krill (i.e., Euphausia superba) rather than smaller species of krill. Acoustic surveys of smaller (15-30 mm) krill may require further validation of both the generalizability and parameterization of applied scattering models.
... At least two other known sources of sound scattering could potentially impact the recorded signal in rivers: micro-organisms (Stanton & Chu, 2000) and micro-structures generated by turbulence. The latter could be, in particular, temperature micro-gradients (Lavery et al., 2003(Lavery et al., , 2013Ross & Lueck, 2003;Seim et al., 1995), or concentration micro-structures of micro-bubbles (Shen & Lemmin, 1997) or fine particles (Merckelbach, 2006). ...
The measurement of suspended sediments from acoustic backscatter was originally developed in marine science for monitoring near‐bottom suspensions usually composed of sand particles. In the last decade, there has been a growing interest in adapting these techniques to rivers. The so‐called “solid particle theory” developed in oceanography mainly applies to suspensions of non‐cohesive solid particles producing incoherent backscatter signal. So far, this theory has been used for interpreting river backscatter even if it relies on assumptions that are not obviously met in rivers. This study uses a set of measurements made on the Rhône River in France to discuss the typical issues which challenge the interpretation of sound backscattering for monitoring suspended sediments in rivers. Large discrepancies between model outputs and measurements for frequencies lower than 2.5 MHz suggest that other scattering processes including flocculation and air micro‐bubbles may have a large impact on acoustic backscatter and attenuation. Deviations of the backscatter echo distribution from Rayleigh statistics were observed, suggesting that the assumption of incoherent backscattering is not always met. This work calls for the development of a more complete theory for interpreting river backscatter.
... (1) may be evaluated analytically or semi-analytically assuming homogeneous material properties and following the approach of Stanton and Chu (2000) to obtain an exact solution. Variations of weakly scattering spheres and finite cylinders were considered, as their solutions have been used as benchmarks for other scattering models for organisms (Jech et al., 2015). ...
As the ecological importance of gelatinous organisms becomes increasingly appreciated, so has the need for improved knowledge of their abundance and distribution. Acoustic backscattering measurements are routine for fisheries assessments but are not yet widely used to survey populations of gelatinous zooplankton. The use of acoustic backscattering techniques to understand the distribution and abundance of organisms requires an understanding of their target strength (TS). This study presents a framework for a sound scattering model for jellyfish based on the Distorted Wave Born Approximation that incorporates size, shape, and material properties of individual organisms. This model, with a full three-dimensional shape rendition, is applied to a common species of scyphomedusa (Chrysaora chesapeakei) and verified experimentally with broadband (52-90 and 93-161 kHz) laboratory TS measurements of live individuals. Cyclical changes in the organism's shape due to swimming kinematics were examined, as well as averages over swimming position and comparisons with scattering from simpler shapes. The model predicts overall backscattering levels and broad spectral behavior within <2 dB. Measured TS exhibits greater variability than is predicted by scaling the size of the organism in the scattering model, showing that density and sound speed vary among individuals.
... Its shape can be modelled as a curved ellipsoid as it is often done for krill species (Chu and Ye, 1999), but more accurate results are obtained by using shapes closer to reality (Stanton et al., 1998). While simple shapes can provide averaged echo levels from aggregations of animals including broadside incidence, accurate individual TS require the finer shape structure and the orientation angle (incident angle with respect to the acoustic wave) (Stanton and Chu, 2000). Information on the acoustic impedance of the animal dictated by the material properties (internal density and soundspeed contrast) is also needed in any case. ...
Highlights
• Swimbladder of Cyclothone spp are non-functional and devoid of gas in the deep ocean.
• Linearly regressed TS from standard length can be employed at 18, 38 and 70 kHz.
• Sound speed and density contrast vary TS level but neutral buoyancy is expected.
• Fish orientation varies both TS level and spectra and is particularly relevant in areas of low numerical density.
• A simple model to estimate width from standard length is also provided.
Abstract
The use of acoustic scattering models provide estimates of single target echoes that allow acousticians to convert acoustic information into biologically meaningful measures. The literature on organisms’ target strength is extensive but is mainly focused on commercial stocks of small pelagic fishes and zooplankton species. A few models of swimbladdered fishes of the mesopelagic zone are also available. However, deep species of the lower mesopelagic and bathypelagic zones tend to have regressed swimbladders or lack one. These habitats have low numerical densities and thus single target studies and angle variation are of particular relevance. Cyclothone spp, the most abundant fishes in the planet and a major constituent of the biomass in the bathypelagic zone, possess gas-filled swimbladders in the upper mesopelagic zone and in larvae stages of all species, but deeper species gradually fill their swimbladder with age. They thus change from a gas-bearing acoustic scattering to a fluid like type. This study applies the Kirchoff Ray Mode (KRM) model based on real fish body shapes of Cyclothone individuals derived from photographs of organisms captured along the year in the Bay of Biscay in order to obtain target strength (TS) of these individuals. Width versus standard length (SL) values fitted the following equation: width=0.01+0.02*SL. Estimated TS values in the Rayleigh zone at broadside had significant linear correlations with SL that can be employed as an approximation of their scattering (TS = 35*log10(SL) − 119, TS = 35*log10(SL) − 106 and TS = 35*log10(SL) − 97 at 18, 38 and 70 kHz respectively). TS at 120 and 200 kHz were not significantly correlated with standard length. Changes in fish body sound speed and density values highly vary the TS level. Assuming neutral buoyancy (body density close to surrounding seawater density), mean TS values were located at −91, −85, −78, −77, −80 dB at 18, 38 and 70, 120 and 200 kHz respectively. TS changes with orientation were also considered depicting important variations in echo level as well as in TS spectra. This study provides relevant information on the acoustic characteristics of lower mesopelagic and bathypelagic Cyclothone species that can be employed to better infer knowledge from acoustic recordings in those areas.
... Biological acousticians have conducted extensive research to distinguish marine organisms using their acoustic backscatter (Benoit-Bird & Lawson 2016). The principles are founded in mathematical acoustic scattering models (Stanton et al. 1998, Stanton & Chu 2000 and have been validated by both laboratory measurements (Wiebe et al. 1990, Stanton et al. 2004) and field testing (Wiebe et al. 1996, Lawson et al. 2001. Received backscatter depends on the acoustic frequency encountering the targets, as well as their shape, orientation, and body structures. ...
Managing human impacts on marine mammal populations depends on understanding their distributions in space and time. Knowledge of these distributions is becoming increasingly important due to offshore energy development and climate-induced shifts in oceanic habitat. Most models assessing pelagic marine mammal abundance and distribution primarily use ocean surface or ocean floor variables as proxies for water column habitat. Here, we used a ship-based marine mammal sighting survey to test the utility of echosounder-based predictive variables for modeling marine mammal distribution and abundance. We assessed the distribution of 7 marine mammal taxa and 3 feeding guilds along the shelf break off the northeast USA relative to prey structure derived from acoustic data. We classified prey into 4 categories: (1) fish with swimbladders; (2) small resonant bubbles as phytoplankton, fish larvae, and gelatinous zooplankton; (3) fluid-like scatterers such as krill and copepods; and (4) fish with no swimbladder. We quantified the spatial structure of prey by calculating backscattering strength, location, dispersion, occupied areas, evenness, and aggregation. Spatial resolution along the survey track line was set to bins 1000 m in distance and either 50 or 200 m in depth. We then built generalized additive models (GAMs) using these acoustically derived variables to explain marine mammal distribution. The resulting GAMs explained between 9 and 38% of deviance, with model fit often reflecting aspects of foraging depth and prey preference. This approach could contribute to improved management through more accurate species distribution models that employ direct measurements of prey.
... To ensure the continuity of the water-column signal, we removed from the dataset any profile with either missing or extremely weak backscatter, the latter probably introduced when cleaning low signal-to-noise regions 60 . The threshold below which a profile was considered to be altered by this pre-processing step was set to −130 dB since no biological backscatter is expected below this level 63 . This threshold permitted removing profiles where the continuity of the signal was compromised by noise conditions without the risk of removing backscatter from animals. ...
Pelagic fauna is expected to be impacted under climate change according to ecosystem simulations. However, the direction and magnitude of the impact is still uncertain and still not corroborated by observation-based statistical studies. Here we compile a global underwater sonar database and 20 ocean climate projections to predict the future distribution of sound-scattering fauna around the world’s oceans. We show that global pelagic fauna will be seriously compromised by the end of the twenty-first century if we continue under the current greenhouse emission scenario. Low and mid latitudes are expected to lose from 3% to 22% of animal biomass due to the expansion of low-productive systems, while higher latitudes would be populated by present-day temperate fauna, supporting conclusions drawn from ecosystem simulations. We further show that strong mitigation measures to contain global warming below 2 °C would reduce these impacts to less than half.
... TS values were used because they were modeled for zooplankton in the same region; however, they were constrained to less than 300 kHz. Because of this constraint, we extrapolated values at 460 kHz using Stanton and Chu (2000) distorted wave borne approximation (DWBA) TS estimates assuming the same form of the scattering curve and using Kitamura et al. to adjust the magnitude of the TS curve (Table I) (Table I). The Delta-SV time series in this study cycled around zero during the cold year summerearly fall months (Fig. 6), which is when Kitamura et al. (2017) collected their samples for TS estimation. ...
A characteristic feature of the eastern Bering Sea (EBS) is a subsurface layer linked to seasonal sea ice (SSI) and defined by bottom temperatures less than 2 °C, which is termed the cold pool. Cold pool variability is directly tied to regional zooplankton and fish dynamics. Multifrequency (200 and 460 kHz) acoustic backscatter data were collected remotely using upward looking echosounders along the EBS shelf from 2008 and 2018 and used as a proxy of biological abundance. Acoustic data were coupled with bottom temperature and regional SSI data from the cold (2006-2013) and warm (2014-2018) regimes to assess the relationship between biological scattering communities and cold pool variation. Acoustic backscatter was 2 orders of magnitude greater during the cold regime than during the warm regime, with multifrequency analysis indicating a shift in the warm regime frequency-dependent scattering communities. Cold pool proxy SSI was a stronger predictor for biological scattering than bottom temperature in the cold regime, while warm regime bottom temperature and SSI were equal in predictive power and resulted in improved predictive model performance. Results suggest coupled cold pool and frequency-dependent scattering dynamics are a potential regime shift indicator and may be useful for management practices in surrounding Arctic ecosystems.
... A gas bladder or pneumatophore contributes more than 95% of the total backscattering strength of an individual organism. Some organisms (C/SNSBF and LNSBF in this study) with densities similar to seawater have weak reflections (Stanton and Chu, 2000;Simmonds and MacLennan, 2005). Accordingly, different scatterers with different sizes, densities, and behavioral characteristics are expected to have different frequency responses among multiple frequencies (Kang et al., 2002(Kang et al., , 2020. ...
In global oceans, ubiquitous and persistent sound scattering layers (SL) are frequently detected with echosounders. The southwest Indian Ocean has a unique feature, a region of significant upwelling known as the Seychelles-Chagos Thermocline Ridge (SCTR), which affects sea surface temperature and marine ecosystems. Despite their importance, sound SL within and beyond the SCTR are poorly understood. This study aimed to compare the characteristics of the sound SL within and beyond the SCTR in connection with environmental properties, and dominant zooplankton. To this end, the region north of the 12°S latitude in the survey area was defined as SCTR, and the region south of 12°S was defined as non-SCTR. The results indicated contrasting oceanographic properties based on the depth layers between SCTR and non-SCTR regions. Distribution dynamics of the sound SL differed between the two regions. In particular, the diel vertical migration pattern, acoustic scattering values, metrics, and positional properties of acoustic scatterers showed two distinct features. In addition, the density of zooplankton sampled was higher in SCTR than in the non-SCTR region. This is the first study to present bioacoustic and hydrographic water properties within and beyond the SCTR in the southwest Indian Ocean.
... 2,8,9 While earlier krill models were only valid for orientations near broadside incidence, 10 scattering models based on a simplified distorted-wave born approximation method were shown to be valid for all angles of orientation. 11,12 Jech et al. 13 analytically and numerically compared the target strength from four simple-shaped structures corresponding to a sphere, a spherical shell, a prolate spheroid and a finite cylindrical shell. Plane wave excitation from different directions and weak scattering boundary conditions were considered. ...
In this paper, we propose non-negative intensity (NNI) as an alternative intensity-based technique for target strength identification in marine ecosystem research. NNI identifies local surface regions of a body with positive-only sound power contributions. NNI is employed for sound scattering by fluid-loaded, fluid-filled elastic structures with weak scattering boundary conditions. Three numerical case studies are presented for which fully coupled fluid-structure interaction models based on the finite element method (FEM) and the boundary element method (BEM) are developed. To validate the three-way coupling between the structural and fluid domains, an elastic shell submerged in water and filled with different internal fluids is initially considered. Results for the scattered acoustic intensity obtained numerically are compared with analytical results from the literature. Models representing Antarctic krill of simple and complex geometry are developed. A 3×3 cylinder array representing a simplified aggregation of krill is also presented. Target strength is calculated using both the scattered intensity and NNI for different incident excitation angles. Results for NNI identify the surface regions of an individual organism or group of organisms with the greatest contribution to the scattered sound at the target strength locations.
... As a result, the structure of r bs for real organisms will typically be less smooth than predicted by the model. To address this, we average r bs over ten angles of incidence normally distributed between À10 and 10 degrees (Stanton and Chu, 2000). Figure 1 shows TS for a representative fluid-like organism. ...
Broadband echosounders measure the scattering response of an organism over a range of frequencies. When compared with acoustic scattering models, this response can provide insight into the type of organism measured. Here, we train the k-Nearest Neighbors algorithm using scattering models and use it to group target spectra (25–40 kHz) measured in the mesopelagic near the New England continental shelf break. Compared to an unsupervised approach, this creates groupings defined by their scattering physics and does not require significant tuning. The model classifies human-annotated target spectra as gas-bearing organisms (at, below, or above resonance) or fluid-like organisms with a weighted F1-score of 0.90. Class-specific F1-scores varied—the F1-score exceeded 0.89 for all gas-bearing organisms, while fluid-like organisms were classified with an F1-score of 0.73. Analysis of classified target spectra provides insight into the size and distribution of organisms in the mesopelagic and allows for the assessment of assumptions used to calculate organism abundance. Organisms with resonance peaks between 25 and 40 kHz account for 43% of detections, but a disproportionately high fraction of volume backscatter. Results suggest gas bearing organisms account for 98.9% of volume backscattering concurrently measured using a 38 kHz shipboard echosounder between 200 and 800 m depth.
... The size and density of the organisms that make up the acoustic scattering layer can be estimated using the difference between the acoustic volume backscattering strengths (SV) of the high and low frequencies [6]. Besides direct echosounder observations, theoretical target strength (TS ) models, such as the Distorted Wave Born Approximation (DWBA) model, can also be used to describe the differences in SV frequency characteristics [2,24]. Previous studies investigating predator and prey overlap were also able to discriminate a number of fish and plankton species using the frequency difference between SVs [18,26,10,12,30]. ...
... The model used was the Distorted Wave Born Approximation (DWBA) model, which is typically used to estimate the acoustic scattering properties of zooplankton such as euphausiids and copepods. This model is effective when the density and the sound speed of the scatterers are close to those of seawater [3,21,26]. The calculation was done for a series of cylindrical slices along the body axis, whereby the scattering amplitude f bs and its relationship with TS were expressed using: ...
It is well known that there is bias using different sampling gear, such as a framed midwater trawl (FMT) and a ring net, when measuring the density and length distribution of target species. This limit is characterized by the sampling efficiency of the gear. Acoustic monitoring can be used to determine the sampling efficiencies of this gear, as its noninvasive, wide-range sweeps provide more reliable estimates of absolute abundance of the target species. The density measured by the gear can then be standardized by multiplying the initial density measurement by the derived sampling efficiency. In this study, we compared the estimated densities of the dominant zooplankton in the sound scattering layer (SSL) from acoustic monitoring to the densities of the same species measured using the FMT and ring net. The respective sampling efficiencies of this gear for zooplankton categories was then determined using linear regression models. For small Euphausiidae and Copepoda, the sampling efficiency of ring net was higher than that of FMT. In contrast, the ring net was less effective than FMT for large Euphausiidae. These results highlight that the entering and retention rates of the species depend on the type and characteristics of the survey gear, as well as the size and swimming ability of the target species.
... The expected target strength of krill can be calculated using target strength models for weakly scattering targets; which krill are considered to be, given their material properties, the sound speed and internal density contrasts compared to the surrounding fluid medium are <15% (Stanton, 2000). There has been considerable development of these models with a view to improving their accuracy. ...
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.
The density and sound-speed contrasts of Antarctic krill in the eastern Indian sector of the Southern Ocean were measured to estimate target strength (TS), which is essential for assessing abundance using acoustic methods. The density contrast of gravid females was significantly smaller than that of individuals at other maturity stages (1.039–1.045). Meanwhile, the sound-speed contrast was the highest in gravid females (1.049), followed by mature females (1.044) and juveniles (1.042). The sound-speed contrast tended to increase with total length. Assuming a body length of 38 mm as the average shape for this study, the TS was calculated using the distorted-wave Born approximation (DWBA) model with the density contrasts and the sound-speed contrasts for each maturity stage. Because the trends compensated each other, the maximum TS at 38 and 120 kHz was −79.9 dB and −64.0 dB, respectively, with no significant difference by maturity stage. In the TS pattern, in which body length and shape were estimated for each maturity stage, the main lobe was broader in gravid females, which differed from the TS pattern for other maturity stages. The effect of the expanded cephalothorax shape of the gravid females was particularly significant.
Shrimp and shelled pteropods are ecologically important organisms that can produce significant amounts of water column backscatter. Few studies have experimentally measured both target strength (TS) and scattering model inputs for individual shrimp and shelled pteropods, especially from the meso- and bathypelagic. We captured animals from net trawls throughout the northeast Pacific and northwest Atlantic continental shelves between 2016 and 2020. We measured morphology, density and sound speed contrasts, and broadband TS (35–75, 110–230 kHz) from tethered individuals at sea and on land in scientific aquaria. Experimentally measured TS was used to test scattering model performance. Differences in measured mean length-to-radius ratios and density contrasts of epipelagic shrimp (10–13 and 1.02–1.03) were statistically greater than for mesopelagic species (8–9 and 1.04–1.05). Differences between predicted and measured shrimp TS averaged over measurement bandwidths were <2 dB, which is similar to previous studies. Conversely, pteropod TS model predictions differed from measurements by >4 dB, with species with elongated shapes having larger differences, likely due to shell shape and uncertainty surrounding material properties. Widely used physics-based acoustic scattering models may significantly underestimate uncertainty in TS predictions for these animals.
The state of the ocean evolves and its dynamics involves transitions occurring at multiple scales. For efficient and rapid interdisciplinary forecasting, ocean observing and prediction systems must have the same behavior and adapt to the ever-changing dynamics. This chapter sets the basis of a distributed system for real-time interdisciplinary ocean field and uncertainty forecasting with adaptive modeling and adaptive sampling. The scientific goal is to couple physical and biological oceanography with ocean acoustic measurements. The technical goal is to build a dynamic modeling and instrumentation system based on advanced infrastructures, distributed/grid computing, and efficient information retrieval and visualization interfaces, from which all these are incorporated into the Poseidon system. Importantly, the Poseidon system combines a suite of modern legacy physical models, acoustic models, and ocean current monitoring data assimilation schemes with innovative modeling and adaptive sampling methods. The legacy systems are encapsulated at the binary level using software component methodologies. Measurement models are utilized to link the observed data to the dynamical model variables and structures. With adaptive sampling, the data acquisition is dynamic and aims to minimize the predicted uncertainties, maximize the optimized sampling of key dynamics, and maintain overall coverage. With adaptive modeling, model improvements dynamically select the best model structures and parameters among different physical or biogeochemical parameterizations. The dynamic coupling of models and measurements discussed here, and embodied in the Poseidon system, represents a Dynamic Data-Driven Applications Systems (DDDAS). Technical and scientific progress is highlighted based on examples in Massachusetts Bay, Monterey Bay, and the California Current System.
For the acoustic inference of lengths and orientation angles of Antarctic krill (krill), reliable theoretical acoustic scattering models and parameters are necessary. To measure the volume backscattering (SV) spectra, and to clarify if the spectral shapes of target strength (TS) predicted by the stochastic distorted-wave Born approximation-based deformed cylinder model (SDWBA-DCM) are in agreement with those of the measured SV, we conducted simultaneous sampling with a broadband echosounder and a rectangular midwater trawl targeting 9 aggregations in the eastern Indian sector of the Antarctic in the 2018/19 austral summer. The SV spectra were measured at 50–85 kHz and 95–255 kHz. Using the SDWBA-DCM and the length frequency distribution obtained from the trawl samples, the length-averaged TS spectra were predicted. Both spectra were normalized by SV and TS values at 120 kHz, respectively, and the relative frequency responses were compared. The spectral shapes were in reasonable agreement in the case of the aggregations dominated by krill smaller than 35 mm. On the other hand, in the case of the krill larger than 35 mm, the spectral shapes were not in agreement. The possible causes of the discrepancy included the orientation angle distribution and the shape of krill.
In two surveys in an Icelandic fjord, September 2016 and October 2018, the target strength (TS) of the euphausiid Thysanoessa raschii was estimated at four frequencies (38, 70, 120, 200 kHz) by matching the acoustic backscatter to the number of euphausiids detected by a Video Plankton Recorder (VPR). Using forward-looking strobe lights on the VPR and doubling the towing speed lowered the estimated target strength by 4.3 dB. In 2016, the TS for euphausiids of mean length 20.7 mm averaged −98.4, −92.3, −86.6 and −82.8 dB at 38, 70, 120 and 200 kHz frequencies, respectively. In 2018, TS for euphausiids of mean length 19.9 mm averaged −98.2 dB at 38 kHz and −88.3 dB at 120 kHz. Theoretical modeling using a Distorted-Wave Born Approximation-based approach was used to compute the average target strength for the observed length distributions and for several density and sound speed contrast (g, h) and orientations. Except at 38 kHz, these results are in reasonable agreement with the TS estimated from the VPR-acoustic comparisons. The methodological approach presented provides an alternative to net-acoustic comparison or modeling for the estimation of euphausiid target strength.
p>The objective of this thesis was to study the distributions of zooplankton at the mesoscale, using acoustic backscatter data (from an Acoustic Doppler Current Profiler, ADCP and SIMRAD EK500 echosounder) taken concurrently with hydrographic data (from SeaSoar and CTDs) and net-sampled zooplankton (from a Longhurst Hardy Plankton Recorder, LHPR). A further aim was to validate acoustic backscatter data with concurrent net data.
Two cruises collected multidisciplinary datasets from two contrasting regions. RRS Discovery cruise 209 to the Arabian Sea (Indian Ocean) in August 1994 and RRS Discovery cruise 224 to the Alboran Sea (Mediterranean Sea) during November and December 1996.
The Arabian Sea was dominated by a strong oxygen minimum zone (OMZ) that extended from the thermocline (~50 m), to ~1100 m. From the analysis of sound scattering layers and net samples the OMZ appeared to play a major role in determining the vertical distribution of zooplankton. A persistent layer of high acoustic backscatter and high zooplankton biovolume indicated that the vertical extent of some zooplankton was restricted to oxygenated surface waters. Euphausiids, decapods and myctophid fish were observed to undertake dawn diel migration between the surface and depths of 300 - 400 m, well into suboxic waters, whilst the vertical distribution of polychaetes appeared to be independent of oxygen concentration and it is suggested that these taxa may possess morphological and physiological adaptations to low oxygen environments.
The Alboran Sea is the site of an intense geostrophic density from that has associated ageostrophic vertical components. It appeared that the front was exhibiting a fertilising effect on both phytoplankton and zooplankton. Observations of a layer of chlorophyll a fluorescence coincident with subducted surface waters indicated that phytoplankton were down and along isopycnals to a depth of ~200 m. Acoustic backscatter data and net samples indicated that increased numbers of euphausiids and chaetognaths occurred coincident with the drawn-down phytoplankton. Smaller zooplankton, copepods and possibly euphausiid larvae, which did not undertake diel migration, remained concentrated near the surface in the fast-flowing frontal jet.</p
We built a new pulse-echo system using a small tank (1 × 1 × 1 m) for measuring the broadband target strength of weakly scattering animals such as krill and shrimp. The system transmits a linear frequency modulated signal with a frequency sweep of 20–220 kHz. To increase the signal-to-noise ratio (SNR), a very long (50 ms) signal is used, and pulse compression processing is applied to received echoes. To determine the accuracy and effectiveness of the system and method, the obtained measurements were compared with predictions by theoretical acoustic scattering models. According to the verification experiment for a sphere and cylinders, the mean absolute errors were < 0.30 dB and the correlation coefficients r were > 0.97 in the frequency range above 20-dB SNR (40–210 kHz for the cylinders). Our measurement system was thus very accurate. We then performed the experiment for a commercially important shrimp, sakura shrimp Lucensosergia lucens. The measured spectra of three samples (35–38 mm) were in good agreement with the predicted spectra using an assumed sound-speed contrast. The r values were > 0.88 in the frequency range above 20-dB SNR (approximately 110–190 kHz). The effectiveness of our new method for weakly scattering animals was confirmed.
Acoustic scattering can be used to estimate Suspended Sediment Concentration (SSC) through acoustic inversion methods. Current SSC quantification methods are mostly unable to observe both spatial and temporal variations. Here, we assess the possibility to measure both using a Multibeam Echosounder (MBES). MBES combine a large spatial covering in the water column and the capability to measure ‘on route’, allowing a better representativity of the measurements. Time-series of raw EM3002-MBES data at 300 kHz were acquired during a 5-hours field experiment at a fixed location in the Aulne macrotidal estuary (France) during ebb, ensuring sufficient SSC variations. Concurrently, 4-frequencies Acoustic Backscattering System (ABS) profiles were acquired in the water column, as well as turbidity profiles, further converted into SSC using collected water samples. An original in-situ calibration was performed on the MBES, using a tungsten sphere of known properties, which allowed corrections to be made to the volume backscattered levels over the echosounder fan. Using ABS-derived equivalent radii, the MBES backscattered signal was inverted to retrieve an SSC estimate. Good consistency between MBES time-series observations and turbidity-derived SSC is observed. This experiment demonstrates the potential use of MBES for 3-dimensional turbidity observations in coastal areas, which is of great interest for sediment flux quantification.
We present the hydroacoustic inversion tool HYDRAC. This versatile open-source
software, developed in Python 3, offers the capability to read hydroacoustic data from widelyused instruments (eg. ABS, ADCP,…) and perform acoustic inversions following several advanced methods found in the literature to estimate the Suspended Particulate Matter mass concentration and particle size characteristics. This software, designed for long-term communitybased developments, includes a specific module for modelling the SPM scattering properties, from organic to mineral particles. The originality of this tool lies in its high technological readiness level (TRL 5), towards the emergence of a performant SPM technology for operational use.
Intermediate trophic components of pelagic ecosystem (from mesozooplakton to micronekton) funnel energy and organic matters from primary producers to many commercial species (anchovy, sardines…). Some of these organisms are supposed to be important contributors to the ubiquitous Sound Scattering Layers (SSLs) observed over a broad range of spatio-temporal scales and geographical areas. Yet, the SSLs taxonomic composition remains largely unknown. The aim of this PhD was to describe the composition and repartition of dense SSLs observed in the Bay of Biscay (France) in spring at several spatial scales, using broadband acoustics, nets and videos. In the first chapter, we showed that the echo sounder beam widths had few effect on the SSLs backscatter, suggesting those SSLs could be composed of unidentified small gaseous scatterers. In the second chapter we applied a forward approach to characterize the composition of SSLs sampled in spring 2016 at small scale, by comparing in situ frequency responses to predictions of scatterer models parameterized with biological sampling data. We determined that these SSLs were probably composed of gas-bearing siphonophores, who, together with mesopelagic fish, dominated the frequency spectra at low frequencies (18-150 kHz). The acoustic backscatter at higher frequencies was dominated by a mix of mesozooplankton organisms, including high densities of pteropods. In the third chapter we applied unsupervised classification methods and a supervised discriminant analysis to delineate the spatial distribution of a peculiar surface SSL composed of centimetric gas-bearing siphonophores. We showed that those small siphonophores were distributed over a meso-scale area (~100 km) in spring 2016 in the northern Bay of Biscay. Our results suggest that siphonophores might be more abundant, and then might play a larger ecological role in eutrophic ecosystems than suspected so far.
The aim of this work was to describe and compare the spatio-temporal patterns and
ecological niches of red squat lobster (Pleuroncodes monodon) and anchovy (Engraulis
ringens) in the Peruvian Sea (MP). Time series of seven indices who describe the
aggregation, abundance and spatial distribution dynamics of both populations were analyzed. With the use of a preference index, it was investigated the dependence of both populations to the water masses of MP. At local scale (~1 nautical mile) ecological niches of both species were characterized using generalized additive models. It was modelled the relationships between the distribution of both species and temperature (TSM), salinity (SSM), dissolved oxygen (OSM), oxycline depth (profOXI), distance to the shelf break (DBP) and macrozooplankton biomass (biomZPK). Results show that, in MP, red squat lobster and anchovy populational dynamics are similar due to six of the seven populational indices was positively correlated. It was determined that red squat lobster (as the same as anchovy) follow the Habitat Based Hypothesis because its populational dynamics are correlated with the fluctuations of its preferred habitat: the Cold Coastal Waters. At local scale, the most influential variables in the distribution of red squat lobster were biomZPK, DBP and profOXI. In the case of anchovy, DBP and profOXI were the most influential variables. Classical abiotic parameters (TSM, SSM and OSM) had very low influence in the distribution of both species. Relationships between the distribution of both species and SSM, OSM and DBP were similar, with red squat lobster distributed more close to the shelf break than anchovy. Relationships with TSM, profOXI and biomZPK differed between species: red squat lobster was distributed in the coolest water available, with absence of macrozooplankton and oxycline depth between 25-40 m. Anchovy was distributed in a broad temperature range, where the oxycline was the shallowest and macrozooplankton biomass was moderate.
A series of investigations were undertaken to observe and describe the soundbackscattering process from larger zooplankton (e u p h a u s i i d s). The target strength versus frequency, size, and aspect angle of the organism was measured. The target strength is highly dependent on the density and sound speed contrasts between the target and the medium, and both these parameters were measured. From the target strength observations it was concluded that the fluid sphere model was insufficient as a scattering model for krill. Observations of the tilt angle distribution of krill at natural field conditions showed that they were distributed over a large region of tilt angles. The deficiencies of the fluid sphere models required the development of a new scattering model based on experimental data. This model predicts a decreasing target strength versus frequency in the geometric scattering region. The backscattering spectra of e u p h a u s i i d s were better described by our empirical model than by the fluid sphere models. Applying the empirical model to estimate size distribution and biomass of krill, we found strong correlation between the acoustically estimated distributions and those from net catches.
The usefulness of bottom-mounted, upward-looking sonars for long-term quantitative zooplankton monitoring was demonstrated utilizing a 49 d record of acoustic volume backscattering strengths collected by a 150 kHz RDI Acoustic Doppler Current Profiler in LaHave Basin off Nova Scotia, Canada. Variations in the character and intensity of 2 distinct acoustic scattering horizons distinguished by differing diurnal migration patterns were noted. One horizon confined to the top 50 m and only present during the early parts of the recording was tentatively ascribed to juvenile fish. Another strongly migrating horizon identified with the euphausiid Meganyctiphanes norvegica was studied in detail. M. norvegica in daytime were broadly distributed at about 150 m depth. At night they were uniformly distributed in the upper 50 m. A 30 % decrease in nighttime integrated backscattering levels was consistent with euphausiid accumulation in the acoustically unobservable near-surface region. Average target strengths declined less-than-or-equal-to 1 dB during active vertical migration. This observation, interpreted by acoustic scattering theory, constrained average euphausiid inclinations to little more than 30-degrees during upward swimming. Column integrated population densities were roughly 290 m-2. Use of an inclined beam sonar geometry should result in backscattering levels less sensitive to organism orientation than use of a conventional vertically oriented sonar beam.
A broadband laboratory chirp sonar was used to investigate the frequency characteristics of the backscattering by two individual
live decapod shrimps (Palaemonetes vulgaris). The measurements illustrate that for fixed shape and orientation, the backscattering amplitude varies dramatically with
frequency with as low as 30-dB-deep nulls at certain frequencies (30 dB corresponds to a factor of 1000 in an echo integration
value). The overall echo level and frequency characteristics vary from ping to ping as the animal changes shape and orientation.
The fluid bent cylinder model [T. K. Stanton (1989), J. Acousl. Soc. Am. 86:691–705] describes the single-realization normal incidence data reasonably well, especially the frequencies at which
the peaks and nulls occur. Our theoretical analysis only involved physical parameters that were either directly measured or
taken from the literature where physical parameters involving similar animals are published. As expected, once the data are
averaged across a number of pings (as one would do in a field measurement), the nulls are mostly “filled in” and the backscatter
versus frequency curve becomes smoother with variability of the order 5–8 dB (factor of 3–6 in echo integration value). These
data are useful for predicting volume scattering strengths observed in the field by taking averages with respect to shape
and orientation.
The theoretical basis for using multiple acoustic frequencies in the assessment of the distributions of small zooplankton (ca. 0.1 mm to 10 mm in length) is discussed. One practical implementation of this theory is the Multifrequency Acoustic Profiling System (MAPS). The MAPS uses 21 discrete frequencies in the band between 100 kHz and 10 MHz. Acoustic data collected with this system are transformed to plots of zooplankton abundance versus size and depth for individual casts. Acoustic estimates of abundance versus size for individual casts are combined to illustrate two-dimensional spatial distribution and temporal variations. These patterns are compared with data collected at the same time for temperature, salinity, and chlorophyll fluorescence. Illustrations from several contrasting environments are included.
Nous discutons les raisons théoriques pour l'utilisation de fréquences acoustiques multiples pour 1'èvaluation des distributions de zooplancton (ca. 0.1 à 10 mm de longueur). Un usage pratique de cette théorie est le Multifrequency Acoustic Profiling System (MAPS). Le MAPS utilise 21 fréquences séparées dans la bande entre 100 kHz et 10 MHz. Nous transformons les données acoustiques récoltées avec ce système en graphes d'abondance de zooplancton contre leurs dimensions et contre la profondeur de mises-a-1è individuelles. Les estimations acoustiques d'abondance contre leurs dimensions pour les mises-à-l'eau individuelles sont combinées pour illustrer la distribution spatiale en deux dimensions et les variations temporelles. Ces résultats sont comparés avec les données de température, de salinité, et de fluorescence de la chlorophylle recueillies simultanément. Des exemples de plusieurs environnements différents sont inclus.
The acoustic echo levels from zooplankton are strongly dependent upon the acoustic frequency and size, shape, orientation, and material properties of the animals. Because of the great number of species of zooplankton, it is practical to study the acoustic properties of species grouped by their gross anatomical similarity. Zooplankton from several major groups are discussed: fluid-like (decapod shrimp, euphausiid, salp), hard elastic shelled (gastropod), and gas-bearing (siphonophore). The results from labora- tory tests show that the plots of (single ping) target strength versus acoustic frequency have a distinct pattern for each animal type. For example, the plot for euphausiids when ensonified at broadside incidence contained a series of broadly spaced deep nulls; the plot for gastropods either had more tightly spaced nulls or aflat spectrum; the plot for siphonophores either had a less consistent pattern of nulls or a flat spectrum. The nulls from the euphausiid data were sometimes as deep as 30 dB below surrounding levels. The patterns are linked to the physics of the scattering process and modeled mathematically. In addition, key results on these animals from Stanton et al. (1994a, ICES J. Mar. Sci., 51: 505-512) are summarized to further illustrate the variability in scattering characteristics of the animal groups (for example, data from 2-mm-long gastropods show that they produce a level of echo energy per unit biomass approxi- mately 19 000 (i.e., 43 dB) times greater than that of 30-mm-long salps). The impact of these observations on design and interpretation of acoustic surveys is discussed. Very importantly, drawing a simple relationship between echo energy and biomass for regions containing a complex assemblage of zooplankton would be greatly flawed. ? 1996 International Council for the Exploration of the Sea
High frequency (420 kHz) sound was used to study the volume backscattering from plankton and micronekton over Georges Bank as part of a study designed to determine the correlation length scales of plankton spatial patterns in relation to physical structure and to intercompare different kinds of sampling and remote-sensing instrumentation. Two physically distinct areas were studied: a well-mixed area in a shallow portion of the Bank and a stratified area on the deeper southern flank of the Bank. A submersible echo sounder with a down-looking transducer was mounted in a towed V-fin. Volume backscattering data were collected from near the sea surface to the bottom (40–80 m).
Measurements and analyses are presented of the backscattering of 420‐kHz sound by 43 individual animals of representative zooplanktonic and micronektonic taxa. Direct measurements of an individual’s target strength were made with a commercial dual‐beam sonar system in an enclosure filled with filtered seawater deployed off a dock at Friday Harbor, Washington. The dependence of target stengths upon individual length, wet weight, and dry weight was investigated. In addition, the ‘‘target strength’’ and statistical variations of echo amplitude due to variations in shape and orientation of the organism were compared with acoustic scatteringmodels involving different shapes (the general shapes of the sphere, and straight and uniformly bent finite cylinders were used along with attempts to take into account roughness). It was found that: (1) backscattering cross sections are proportional to volume of the organisms rather than area as would be predicted by a sphere scatteringmodel, (2) mean target strength based on average backscattering crossection is best described by the bent cylinder model whose modal series solution is truncated, and (3) the fluctuations of the echo amplitudes are well described by the Rice probability density function whose shape parameter is related to the randomly rough straight cylinder model. These extensive studies showed conclusively that the elongated animals scatteredsound more like elongated targets than spherical ones, thus demonstrating the need for models more sophisticated than the spherical ones routinely used to date. The data and modelanalyses provide a basis for devising future acoustical data acquisition and processing techniques for bioacoustical field studies.
Author Posting. © Acoustical Society of America, 1993. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 93 (1993): 2985-2988, doi:10.1121/1.405818. Data collected during the krill target strength experiment [J. Acoust. Soc. Am. 87, 16–24 (1990)] are examined in the light of a recent zooplankton scattering model where the elongated animals are modeled as deformed finite cylinders [J. Acoust. Soc. Am. 86, 691–705 (1989)]. Exercise of the model under assumption of an orientation distribution allows absolute predictions of target strength to be made at each frequency. By requiring that the difference between predicted and measured target strengths be a minimum in a least-squares sense, it is possible to infer the orientation distribution. This useful biological quantity was not obtainable in the previous analysis which involved the sphere scattering model. This research was supported, in part, by the Ocean Acoustics and Oceanic Biology Program of the Office of Naval Research Grant No. N00014-89-J-1729
Author Posting. © Acoustical Society of America, 1993. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 94 (1993): 3454-3462, doi:10.1121/1.407199. Data indicate that certain important types of marine organisms behave acoustically like weakly scattering fluid bodies (i.e., their material properties appear fluidlike and similar to those of the surrounding fluid medium). Use of this boundary condition, along with certain assumptions, allows reduction of what is a very complex scattering problem to a relatively simple, approximate ray-based solution. Because of the diversity of this problem, the formulation is presented in two articles: this first one in which the basic physics of the scattering process is described where the incident sound wave is nearly normally incident upon a single target (i.e., the region in which the scattering amplitude is typically at or near a maximum value for the individual) and the second one [Stanton et al., J. Acoust. Soc. Am. 94, 3463–3472 (1993)] where the formulation is heuristically extended to all angles of incidence and then statistically averaged over a range of angles and target sizes to produce a collective echo involving an aggregation of randomly oriented different sized scatterers. In this article, a simple ray model is employed in the deformed cylinder formulation [Stanton, J. Acoust. Soc. Am. 86, 691–705 (1989)] to describe the scattering by finite length deformed fluid bodies in the general shape of elongated organisms. The work involves single realizations of the length and angle of orientation. Straight and bent finite cylinders and prolate spheroids are treated in separate examples. There is reasonable qualitative comparison between the structure of the data collected by Chu et al. [ICES J. Mar. Sci. 49, 97–106 (1992)] involving two decapod shrimp and this single-target normal-incidence theory. This analysis forms the basis for successful comparison (presented in the companion article) between the extended formulation that is averaged over an ensemble of realizations of length and angle of orientation and scattering data involving aggregations of up to 100's of animals. This work was supported by the U.S. Office of Naval Research Grant No. N00014-89-J-1729 and National Science Foundation Grant No. OCE-8817171.
By heuristically extending the previously developed ray solution [Stanton et al. J. Acoust. Soc. Am. 94, 3454–3462 (1993)] to predict the scattering by cylinders over all angles of incidence, approximate expressions are derived which describe the echo energy due to soundscattered by finite cylinders averaged over orientation and length. Both straight and bent finite length cylinders of high aspect ratio are considered over the full range of frequencies (Rayleigh through geometric scattering). The results show that for a sufficiently broad range of orientation, the average echo is largely independent of the degree of bend—that is, the results are essentially the same for both the straight and bent cylinders of various radii of curvature (provided the bend is not too great). Also, in the limit of high frequency (i.e., the acoustic wavelength is much smaller than the cross‐sectional radius of the object), the averages are independent of frequency. The resultant formulas derived herein are useful in describing the scattering by elongated zooplankton whose shape may not necessarily be known in the natural ocean environment. The average echo is shown to depend directly upon standard deviation (s.d.) of the angle of orientation as well as size. If independent measurements of size are made (such as from trawling samples), then the properties of the angle distribution and hence behavior may be inferred from the data. Averages over both angle and a narrow distribution of size are shown to only partially smooth out deep nulls in the scatter versus frequency curves. The formulas compare favorably with laboratory data involving aggregations of animals and a broad range of frequencies (38 kHz to 1.2 MHz).
A modeling study was conducted to determine the conditions under which fluidlike zooplankton of the same volume but different shapes (spherical/cylindrical) have similar or dramatically different scattering properties. Models of sound scattering by weakly scattering spheres and cylinders of finite length used in this analysis were either taken from other papers or derived and herein adapted for direct comparison over a range of conditions. The models were examined in the very low- (ka < 1, kL < 1), moderately low- (ka < 1, kL > or = 1), and high-frequency regions (ka > 1, kL > 1), where k is the acoustic wave number, a is the radius (spherical or cylindrical) of the body, and L is the length of the cylinders (for an elongated body with L/a = 10, "moderately low" corresponds to the range 0.1 < or = ka < or = 0.5). Straight and bent cylinder models were evaluated for broadside incidence, end-on incidence, and averages over various distributions of angle of orientation. The results show that for very low frequencies and for certain distributions of orientation angles at high frequencies, the averaged scattering by cylinders will be similar, if not identical, to the scattering by spheres of the same volume. Other orientation distributions of the cylinders at high frequencies produce markedly different results. Furthermore, over a wide range of orientation distributions the scattering by spheres is dramatically different from that of the cylinders in the moderately low-frequency region and in the Rayleigh/geometric transition region: (1) the Rayleigh to geometric scattering turning point occurs at different points for the two cases when the bodies are constrained to have the same volume and (2) the functional dependence of the scattering levels upon the volume of the bodies in the moderately low-frequency region is quite often different between the spheres and cylinders because of the fact that the scattering by the cylinders is still directional in this region. The study demonstrates that there are indeed conditions under which different shaped zooplankton of the same volume will yield similar (ensemble average) scattering levels, but generally the shape and orientation distribution of the elongated bodies must be taken into account for accurate predictions.
Mathematical scattering models are derived and compared with data from zooplankton from several gross anatomical groups--fluidlike, elastic shelled, and gas bearing. The models are based upon the acoustically inferred boundary conditions determined from laboratory backscattering data presented in part I of this series [Stanton et al., J. Acoust. Soc. Am. 103, 225-235 (1998)]. The models use a combination of ray theory, modal-series solution, and distorted wave Born approximation (DWBA). The formulations, which are inherently approximate, are designed to include only the dominant scattering mechanisms as determined from the experiments. The models for the fluidlike animals (euphausiids in this case) ranged from the simplest case involving two rays, which could qualitatively describe the structure of target strength versus frequency for single pings, to the most complex case involving a rough inhomogeneous asymmetrically tapered bent cylinder using the DWBA-based formulation which could predict echo levels over all angles of incidence (including the difficult region of end-on incidence). The model for the elastic shelled body (gastropods in this case) involved development of an analytical model which takes into account irregularities and discontinuities of the shell. The model for gas-bearing animals (siphonophores) is a hybrid model which is composed of the summation of the exact solution to the gas sphere and the approximate DWBA-based formulation for arbitrarily shaped fluidlike bodies. There is also a simplified ray-based model for the siphonophore. The models are applied to data involving single pings, ping-to-ping variability, and echoes averaged over many pings. There is reasonable qualitative agreement between the predictions and single ping data, and reasonable quantitative agreement between the predictions and variability and averages of echo data.
Acoustical scattering from aquatic organisms is an extremely complicated problem and can only be dealt with by empirical or semi-empirical models. In order to use sonars as a remote-sensing tool to study biological sound scatterers in the ocean, one must rely on such models. Our research at the Gulf Stream Boundary near Cape Hatteras, North Carolina, involves studying heterogeneous distributions of fish, crustaceans (e.g., shrimp), cephalopods (e.g., squid), and soft-bodied animals (e.g., salps) and their relationship to the front. To quantify the scattering from the animals, we use existing scattering models. However, the models were based on data from animals in artificial environments such as a laboratory tank or an underwater cage near a ship. Since scattering strengths depend on the animal's behavior, depth, and whether it is alive or dead (and possibly preserved), data from the artificial environments may not accurately represent the scattering from the animals in their natural environments. Furthermore, the scattering models are mainly based on species not found in the area studied; therefore, we have considered them as general models for selected taxonomic groups. Working under this assumption, the present study examines the robustness of the general models to a field application. We used size frequency distribution of the various animal types from trawl data as input into the scattering models to produce a predicted value of the acoustic volume scattering strength. We also examined the field sonar data for potential problems in comparing trawl and sonar data (e.g., large animals which have high target strengths can avoid trawls). Our comparison between the predicted and field measured values demonstrates the potential usefulness of the general models in areas with different taxonomic assemblages. We find most data to lie within 5 dB of the predictions with some data systematically outside the 5-dB bars, which indicates animals at some stations were avoiding the trawl.
Acoustic scattering experiments involving simultaneous acquisition of broadband echoes and video footage from several Antarctic krill were carried out to determine the effect of animal orientation on echo spectral structure. A novel video analysis technique, applied to extract krill angle of orientation corresponding to each insonification, revealed that echo spectra from krill near broadside incidence relative to the incident acoustic wave exhibited widely spaced, deep nulls, whereas off-broadside echo spectra had a more erratic structure, with several closely spaced nulls of variable depth. The pattern of changes in echo spectra with orientation for the experimentally measured acoustic returns was very similar to theoretically predicted patterns based on a distorted wave Born approximation (DWBA) model. Information contained in the broadband echo spectra of the krill was exploited to invert the acoustic returns for angle of orientation by applying a newly developed Covariance Mean Variance Classification (CMVC) approach, using generic and animal-specific theoretical and empirical model spaces. The animal-specific empirical model space was best able to invert for angle of orientation. The CMVC inversion technique can be implemented using a generic empirical model space to determine angle of orientation based on broadband echoes from individual zooplankton in the field.
Accurate acoustic characterization of zooplankton species is essential if reliable estimates of zooplankton biomass are to be made from acoustic backscattermeasurements of the water column. Work on the forward problem has resulted in the identification of three categories of scatterers, represented by theoreticalscatteringmodels [Stanton et al., ICES J. Marine Sci. 51, 505?512 (1994)]: gas?bearing (e.g., siphonophores), fluidlike (e.g., euphausiids), and hard elastic?shelled (e.g., pteropods). If there are consistent differences in the characteristic acoustic signatures of each of these classes of zooplankton, it should be possible to solve the inverse problem by using acoustic backscatter data to mathematically infer the class of scatterer. Two different inversion techniques are applied to a dataset of several hundred pings collected from broadband insonifications (?350?750 kHz) of live zooplankton tethered and suspended in a large tank filled with filtered seawater. The model?based classifiers depend on comparison of the data with theoreticalscatteringmodels, whereas the feature?based classifiers are independent of the models and exploit only the inherent characteristics of the acoustic returns. Preliminary results indicate that the acoustic classification of zooplankton insonifications into categories representing distinct scatterer?types is feasible.
The backscattering frequency responses for euphausiids and copepods are predicted using a fluid‐sphere model and measured physical properties for the zooplankters. The fluid‐sphere model is also compared with the resonant gas bubble equation.
The scattering of sound by objects is a complex process and is dealt with both analytically and numerically. Regardless of method, calculation of the exact scattered field is quite laborious and may require much computer time as well as human time to program the computer. In order to simplify calculations of the backscattering by a fluid sphere, Johnson combined low‐ and high‐frequency limits in a heuristic manner to obtain a simple closed‐form solution (named the “high‐pass” solution) [J. Acoust. Soc. Am. 61, 375–377 (1977)]. In this present work, his approach is refined and extended to the prolate spheroid, straight finite cylinder, and bent finite cylinder. The high‐pass solutions are formulated in a general way so as to describe scattering by idealized objects where their shape is simple (i.e., sphere, spheroid, etc.) and their material is lossless and by nonideal more realistic objects where their shape may be irregular and their material lossy. Comparisons are made of the scattering of these objects using both modal series solutions and the “high‐pass” solutions for fluid, elastic, rigid and fixed, and gaseous materials over a wide range of frequencies. In addition to the numerical simulations, some of the high‐pass solutions and corresponding modal series solutions are compared with data involving “ideal” objects (machined Dural) and “nonideal” objects (marine organisms: shrimp, euphausiids, and fish). The numerical and experimental results show promise for use of the high‐pass models for quick estimates of backscattered sound of many types of objects. [Work supported by the ONR.]
Measuredbackscattering spectra for preserved specimens of three zooplankters (copepod, euphausiid, sergestid shrimp) are compared to fluid sphere scattering models. Quantitative agreement is found at low k a for all species, however the copepod results are increasingly higher than predicted for k a≳1 and the euphausiid and sergestid shirmp are found to be directional scatterers. A fluid prolate spheroid model is proposed fo the latter species.
The speed of longitudinal sound waves in Antarctic krill has been measured by the time-of-flight method. The result of 17 separate measurement series on different assemblages of living krill is that the animal's sound speed exceeds that of seawater at the same temperature by 2.79 ± 0.24%. The mean lengths vary from 29.4 to 38.9 mm, with overall mean 32.2 and s.d. 2.5 mm. The corresponding density of krill of mean length 31 mm is 1.0647±0.0069 g/cm3. Measurement temperatures varied from 5.3 to 12.1°C; corresponding salinities varied from 32.5 to 33.87 ppt, which also represent the ambient state. The ambient sea temperature was 2.0±0.3°C.
Encaged aggregations of live krill in good to pristine condition have been ensonified at 38 and 120 kHz. Concurrent underwater television observations of behavior resemble those made by underwater divers in naturally occurring swarms, with comparably high densities of the order of 104 animals/m3 . Mean, single‐animal target strengths have been inferred from measurements of echo energy. For aggregations with mean lengths in the range [30,39] mm, the mean single‐krill target strengths are in the range [−88,−83] dB at 38 kHz and [−81,−74] dB at 120 kHz. Collateral measurements on some of the same encaged specimens determined a density contrast of 1.0357±0.0067 and sound‐speed contrast of 1.0279±0.0024, relative to seawater. These numbers have been used with the fluid‐sphere model as stated by Greenlaw [Limnol. Oceanogr. 2 4, 226–242 (1979)] . Computed backscattering cross sections have been averaged over the length distributions of each measuredaggregation, resulting in target strength predictions in the range [−86,−80] dB at 38 kHz and [−79,−76] dB at 120 kHz.
There are many different finite length scatterers in the ocean such as marine biota in the water column and protuberances (or "bosses") on the seafloor, sea surface, and underside of sea ice. Since it is impossible to describe scattering from the exact shape of most objects, one must use simple geometrical objects such as spheres, spheroids, or finite cylinders as the approximate shape. In this article, the scattering of an incident plane wave by a fluid finite circular cylinder is described for all frequencies. By neglecting end effects, the volume flow per unit length of the scattered field of the finite cylinder is approximated by that of the infinite cylinder. The solution is obtained by integrating this volume flow along the length of the cylinder. This approximation restricts the solution to geometries where the incident waves are normal or near normal to the axis of the cylinder. The solution is adapted to describe the scattering of sound by shrimp, which are elongated fluid-saturated marine organisms. There was excellent agreement between the adapted solution and shrimp backscatter data.
The swimming angle of isada krill (Euphausia pacifica Hansen) was measured in a tank and the target strength (TS) values were calculated using a theoretical scattering model.
The average swimming angle was 30.4° (s.d.=19.9°), which was about 15° less than that reported for Antarctic krill (Euphausia superba). Parameters for the swimming angle distribution were substituted into the straight cylinder model and the corresponding
TS values were determined. The mean TS values of 16.4 mm isada krill for hovering animals were lower than the maximum TS;
differences were 7.8–5.0 dB, 14.8–8.6 dB, and 18.1–10.8 dB at 50, 120, and 200 kHz, respectively. These differences will lead
to a 1/6–1/3, 1/30–1/7, and 1/65–1/12 proportional difference, respectively, in estimating abundances compared to estimates
based on the maximum TS. Measurements carried out at higher frequencies are more influenced by swimming angle than those at
lower frequencies. However, the TS is less at lower frequencies and it is more difficult to filter out noise. Thus, medium
frequencies are suggested as the best for krill surveys. Additionally, when conducting resource surveys by echosounding, it
is necessary to assign the appropriate swimming angles for different targets. Swimming angles need to be investigated further
under a variety of conditions.
The use of high-frequency acoustic scattering techniques has become central to the study of bilogical and physical oceanography,
yet quantitative discrimination between sound scattered by zooplankton and turbulence has long eluded researchers. (Turbulence
in the presence of a temperature gradient gives rise to variations in sound speed which in turn scatter sound.) Our calculations
indicated that the target strength of a 2-cm-long single shrimp can be comparable to the scattering level from 1 m3 of turbulence in the ocean. Because of the potential ambiguities in acoustic remote sensing of the two type of scatterers,
laboratory measurements were made of the acoustic scattering properties of a small patch of turbulence and a 2-cm-long decapod
shrimp over a frequency range of 300–800 kHz. The data were also used to emulate echoes that one might expect in the ocean
environment. The spectrum of a single echo of the laboratory turbulence and its time evolution was quite irregular and different
than the more stable spectra of the echoes from the individual animals near broadside incidence which contained consistent
major peaks and nulls. There were also noticeable differences between incoherent averages of the echoes as the averaged spectrum
of the turbulence echo was observed to remain irregular (although with reduced variability), while that of the animals retained
some regularity. These results indicate the potential for discriminating between turbulent fields and single animals.
A general solution is derived for the scattering of sound by cylinders of finite length with a deformed axis and composition profile and (cross-sectional) radius that vary along the axis. The orientation of the axis, plane-wave source direction, and point receiver position can also vary as long as the directions of incident and scattered fields are nearly perpendicular to the tangent of the axis (this restfiction can be relaxed under some conditions). This approximate solution is a generalization of previous work [T. K. Stanton, $. Acoust. Soc. Am. 83, 55-63 (1988); and 83, 64-67 (1988) ] where the scattering by straight finite cylinders of uniform fluid and elastic material, respectively, was described. In those articles, the volume flow per unit length of the scattered field of the cylinders was held constant along the length of the axis that restricted the axis to be straight and composition profile and (cross-sectional) radius to remain constant along the length of the axis. In this article, the volume flow per unit length is allowed to vary, thus allowing the above-mentioned quantities also to vary. As a result, an integral equation is derived that, in general, needs to be evaluated numerically. Examples are given in this article of the scattering of sound at all frequencies from a prolate spheroid with a high aspect ratio (i.e., high ratio of major axis to minor axis) and a uniformly bent finite cylinder of constant cross-sectional radius. There is excellent agreement between the (deformed cylinder) calculations involving the prolate spheroid and the exact spheroidal wave-function solution. Furthermore, numerical integration of the deformed cylinder formula required far less computer time than calculating the exact solution. Calculations involving the bent cylinder are compared to backscatter data from preserved euphausiids and suggest that the radius of curvature of the animals plays a major role in the acoustic scatter characteristics of the marine organisms. For example, at 200 kHz, the backscattering cross section of a 23-mm-long euphausiid will decrease by 6 dB if the animal bends by as little as 1.4 mm at the ends.
High-frequency acoustical technology is used to measure the spatial distributions of small zooplankton with meter-scale resolution in the vertical, resolutions of hundreds of meters or better in the horizontal, and with temporal resolutions of minutes. Both size and abundance measurements can be made by using sensors in a cast mode from a ship, from undulating towed bodies and by placing instruments on moorings. This presentation includes an overview of some of the technical concepts that are involved, examples of the use of the sensors and systems that have been developed and brief discussions related to the algorithms used to convert acoustical measures to biologically relevant estimates for small zooplankton. (C) 1995 International Council for the Exploration of the Sea
Acoustic echo sounders have been used relatively successfully for several decades in the detection and possible classification
of simple population (i.e. single size, single species) of fish. One assumption used in these studies is that the energy of
the acoustic echo from a school of fish is related to the biomass of the animals via a simple linear regression curve. However,
as a result of the natural species diversity within zooplankton aggregations, use of acoustic methods to quantify the populations
of zooplankton represents a challenge because the acoustic scattering properties of each gross anatomical class of zooplankton
are quite unique. As a result, large errors can occur if one relies on a simple regression curve to describe the echo energy/biomass
relationship. Because of the great importance of understanding variability of echo levels due to changes in anatomical features
of these animals, we embarked on an experimental investigation in which ship-board and land-based laboratory measurements
of echo levels were made on freshly caught zooplankton. Our results indicate variation by factor of about 19 000 to 1 (43
dB) in the relative average echo energy per unit biomass due to animals ranging from fluid-like (salps) to elastic-shelled
(gastropods).
Acoustical estimates of zooplankton abundance can be made rigorously if the scattering behavior as a function of size and frequency for the zooplankters is known. Measurements of scattering at a single frequency can be used to estimate abundance if the mean zooplankter size is known. Measurements at two frequencies can be used to estimate the dominant size as well as abundance if a single size zooplankter dominates the acoustical scattering. Measurements at several frequencies can be used to estimate size distributions and abundances. In a field experiment, acoustical scattering was measured at three frequencies for zooplankton layers composed largely of euphausiids (for which an approximate scattering model is known). These data are analyzed by each method and estimates of numerical abundance given.
The scattering of sound from a spherical fluid obstacle of size comparable to a wave‐length is considered, neglecting dissipation. Calculations of the acoustic pressure and the total energy in the scatteredwave are presented graphically; soundvelocities and densities of the sphere lie between 0.5 and 2.0 times that of the external medium. The limiting cases of Rayleigh scattering and scattering from a fixed rigid sphere are also shown for comparison. In the region where the diameter of the sphere is comparable to a wave‐length, the scattering is a complicated function of frequency, showing in some cases large maxima and minima. The amplitude of the scatteredwave in the backward direction from a fluid sphere a few wave‐lengths in diameter exceeds twice that from a rigid sphere of the same size for the case of the soundvelocity 0.8 and density equal to that of the surrounding medium.
The width of the main lobe of the acoustic backscatter directivity pattern of decapod shrimp (Palaemonetes vulgaris) is examined versus acoustic frequency. Using the distorted wave Born approximation (DWBA) and the geometry of a prolate spheroid, an analytic formula for the backscatter cross section as a function of orientation angle is derived. A directivity pattern is determined from the analytic formula and the width of the main lobe (beamwidth) is computed. The relationship between beamwidth and acoustic frequency is presented in plots of beamwidth versus ka and L/lambda. The model is adapted to experimental limitations of animal motion, discrete sampling and observed side lobe levels. The backscatter directivity patterns of live decapod shrimp. determined experimentally at frequencies between 72 and 525 kHz, are presented. A non-monotonic relationship between beamwidth and frequency is illustrated in this study. This relationship is in contrast to the monotonic relationship exhibited when sound scatters off of an impenetrable flat plate. Reasonable agreement is found between the theoretically predicted beamwidths and most experimental data. Where the beamwidth was more-or less oscillatory about a mean value of 19 deg. The structure can at least be partly explained by scattering theory.
The density of a lipid mixture composed of nearly 85% wax esters and isolated from the copepod Calanus plumchrus was determined at 5.13, 14.49, and 23.65°C and at pressures up to 783 bars. At atmospheric pressure, the density was determined at temperatures between 1.64 and 15.54°C. The experimental isotherms and isobars were fitted to several equations of state and these were used to calculate coefficients of thermal expansion and compressibility. The wax ester mixture visibly melted as a function of temperature at atmospheric pressure. The high pressure isotherms show a volume discontinuity as a function of pressure that suggests a first-order liquid-solid phase transition. The coefficient of thermal expansion at atmospheric pressure is quite large (up to ten times that of water) and is apparently due in part to the phase change and its concomitant volume change. The coefficient of compressibility at atmospheric pressure is larger for the more solid form of the wax ester mixture than for the more liquid form. The contribution of wax esters to the buoyant properties of organisms is discussed. The buoyant force on the wax ester in seawater changes more as a function of temperature than as a function of pressure. The data also have implications to understanding sound scattering by wax ester-containing organisms. At depths defined by the pressure-temperature curve for the phase change, the wax ester mixture is on the verge of either melting or freezing. This results in a large coefficient of compressibility and may result in distinctive sound scattering by organisms containing wax esters and swimming at such depths. Furthermore, sound scattering should be different in organisms with solid wax ester mixtures than in those with liquid mixtures.
Backscattering measurements of 14 live individual Antarctic krill (Euphausia superba) were made at a frequency of 120 kHz in a chilled insulated tank at the Long Marine Laboratory in Santa Cruz, CA. Individual animals were suspended in front of the transducers, were only loosely constrained, had substantial freedom to move, and showed more or less random orientation. One thousand echoes were collected per animal. Orientation data were recorded on video. The acoustic data were analyzed and target strengths determined from each echo. A method was developed for estimating the three-dimensional orientation of the krill based on the video images and was applied to five of them, giving their target strengths as functions of orientation. Scattering models based on a simplified distorted-wave Born approximation (DWBA) method were developed for five animals and compared with the measurements.Both measured and modeled scattering patterns showed that 120 kHz acoustic scattering levels are highly dependent on animal orientation. Use of these scattering patterns with orientation data from shipboard studies of E. superba gave mean scattering levels approximately 12 dB lower than peak levels. These results underscore the need for better in situ behavioral data to properly interpret acoustic survey results. A generic E. superba DWBA scattering model is proposed that is scalable by animal length. With good orientation information, this model could significantly improve the precision and accuracy of krill acoustic surveys.
Acoustic scattering experiments involving simultaneous acquisition of broadband echoes and video footage from several Antarctic krill were carried out to determine the effect of animal orientation on echo spectral structure. A novel video analysis technique, applied to extract krill angle of orientation corresponding to each insonification, revealed that echo spectra from krill near broadside incidence relative to the incident acoustic wave exhibited widely spaced, deep nulls, whereas off-broadside echo spectra had a more erratic structure, with several closely spaced nulls of variable depth. The pattern of changes in echo spectra with orientation for the experimentally measured acoustic returns was very similar to theoretically predicted patterns based on a distorted wave Born approximation (DWBA) model. Information contained in the broadband echo spectra of the krill was exploited to invert the acoustic returns for angle of orientation by applying a newly developed Covariance Mean Variance Classification (CMVC) approach, using generic and animal-specific theoretical and empirical model spaces. The animal-specific empirical model space was best able to invert for angle of orientation. The CMVC inversion technique can be implemented using a generic empirical model space to determine angle of orientation based on broadband echoes from individual zooplankton in the field. (C) 1998 Acoustical Society of America. [S0001-4966(98)05109-1]
The deformed cylinder method is a very versatile model that can be used on a wide range of acoustic scattering problems. It may be applied to any axisymmetric body, whether it be rigid, an elastic solid, or a shell. Previous results obtained from this model have been restricted to broadside backscattering. This work is the first to verify the validity of the deformed cylinder method over such a wide range of shapes and material properties for angles close to broadside incidence. In order to determine the angular range from broadside over which the deformed cylinder method is valid, these estimates are compared with approximation from other techniques currently available. Agreement at angles of up to 30ø from broadside was obtained for the rigid targets considered. The paper also shows that the deformed cylinder method produces reasonable levels for solid elastic bodies for up to 10 ø to 20 ø off normal incidence.
For inversion problems in which the theoretical relationship
between observed data and model parameters is well characterized, a
promising approach to the classification problem is the application of
techniques that capitalize on the predictive power of class-specific
models. Theoretical models have been developed for three zooplankton
scattering classes (hard elastic-shelled, e.g., pteropods; fluid-like,
e.g., euphausiids; and gas-bearing, e.g., siphonophores), providing a
sound basis for model-based classification approaches. The covariance
mean variance classification (CMVC) techniques classify broad-band
echoes from individual zooplankton based on comparisons of observed echo
spectra to model space realizations. Three different CMVC algorithms
were developed: the integrated score classifier, the pairwise score
classifier, and the Bayesian probability classifier; these classifiers
assign observations to a class based on similarities in covariance,
mean, and variance while accounting for model spare ambiguity and
validity. The CMVC techniques were applied to broad-band (~350-750 kHz)
echoes acquired from 24 different zooplankton to invert for scatterer
class and properties. All three classification algorithms had a high
rate of success with high-quality high SNR data. Accurate acoustic
classification of zooplankton species has the potential to significantly
improve estimates of zooplankton biomass made from ocean acoustic
backscatter measurements
Comparing FE/BE models with measurement: flextensional transducers
- D T I Francis
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Francis, D. T. I., Oswin, J. R., and Macey, P. C. 1996.
Comparing FE/BE models with measurement: flextensional
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31-40.
The biology of cold-core rings. Oceanus
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Wiebe, P. H. 1976. The biology of cold-core rings. Oceanus, 19: 69–76.
Backscattering of ultrasonic waves by shrimps
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Broadband acoustic scattering signatures of fish and zooplankton (project BASS)
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Foote, K. G. 1998. Broadband acoustic scattering signatures
of fish and zooplankton (project BASS). Proceedings of
the Third European Marine Science and Technology
Conference, Lisbon, May 23-27, 3: 1011-1025.