Target Strength Model Research Articles

Experimental target strength measurements of pteropods and shrimp emphasize the importance of scattering model inputs

Abstract 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.

Acoustic Target Strength of Jellyfish, Nemopilema nomurai, Measured at Multi-Frequency and Multi-Orientation

The jellyfish Nemopilema nomurai occupies an important position in the Northwest Pacific ecosystem, and monitoring its biomass is necessary to ensure the protection of fishery resources and the safety of offshore industrial production. The acoustic method has been proposed for jellyfish flux estimation and bloom warning, in which the target strength (TS) of the jellyfish is a crucial parameter. However, varied swimming orientations of jellyfish aggregation result in different backscatter strengths. The acoustic echo characteristics in horizontal swimming orientations and multi-frequency broadband signals are yet to be revealed. This study aims to obtain the TS of jellyfish in various orientations and to comprehensively investigate the jellyfish acoustic echo features at various frequency broadband sounds. In an anechoic tank, we used wide-band echosounder and 70 kHz, 120 kHz, and 200 kHz split-beam transducers to measure the TS of jellyfish swimming omnidirectionally. The results show a difference of approximately 4 dB in the jellyfish’s normalized TS at 70 kHz (frequency range: 45 kHz to 95 kHz) and 200 kHz (frequency range: 160 kHz to 260 kHz) center frequency. The normalized TS of jellyfish varies by around 8 dB between horizontal and vertical swimming orientations. For jellyfish swimming horizontally, the TS and bell diameter have the following least squares fits: T S D 70 k H z = 20 log D − 89.36 r 2 = 0.83 ; T S D 200 k H z = 20 log D − 93.85 r 2 = 0.83 . The swimming orientation has significant effects on TS estimation and model construction.

Environmental variability and krill abundance in the central California current: Implications for ecosystem monitoring

Krill are a direct conduit between primary productivity and recreationally and commercially important higher trophic level species globally. Determining how krill abundance varies with temporal environmental variation is key to understanding their function in coastal-pelagic food webs, as well as applications in fisheries management. We used nine years (2012–19 and 2021) of late spring/early summer hydroacoustic-trawl survey data in the California Current Ecosystem (CCE), coupled with new target strength models of two krill species (Euphausia pacifica and Thysanoessa spinifera), to investigate how adult krill biomass varied during a decade of unusual ocean climate variability. We estimate a mean biomass of 1.75–2.0 million metric tons on the central and northern California continental shelf. Overall, relative krill biomass was ~30% lower during 2015 and 2016, corresponding to a major warming event, and ~30% higher in 2013 and 2018, years of exceptionally strong upwelling. Variation in biomass was related to the prior year’s environmental conditions derived from our seasonal Multivariate Ocean Climate Index (MOCI), and E. pacifica and T. spinifera showed similar covariation during the study period. Biomass co-varied at different spatial scales and across sampling devices, suggesting that multiple indicators of abundance (and dispersion) are available and should be applied in ecosystem monitoring and modeling of krill and krill-dependent predators in the California Current ecosystem.

Krill biomass estimation: Sampling and measurement variability

Krill are the subject of growing commercial fisheries and therefore fisheries management is necessary to ensure long-term sustainability. Krill catch limits, set by Commission for the Conservation of Antarctic Marine Living Resources, are based on absolute krill biomass, estimated from acoustic-trawl surveys. In this work, we develop a method for determining an error budget for acoustic-trawl surveys of krill which includes sampling and measurement variability. We use our error budget method to examine the sensitivity of biomass estimates to parameters in acoustic target strength (TS) models, length frequency distribution and length to wetmass relationships derived from net data. We determined that the average coefficient of variation (CV) of estimated biomass was 17.7% and the average CV due from scaling acoustic observations to biomass density was 5.3%. We found that a large proportion of the variability of biomass estimates is due to the krill orientation distribution, a parameter in the TS model. Orientation distributions with narrow standard deviations were found to emphasise the results of nulls in the TS to length relationship, which has to potential to lead to biologically implausible results.

Combined video–hydroacoustic survey of nearshore semi-pelagic rockfish in untrawlable habitats

Abstract New survey technologies are needed to survey untrawlable habitats in a cost-effective and nonlethal manner with minimal impacts on habitat and nontarget species. Here, we test the efficacy of integrating data from a suspended underwater camera with acoustic data to generate population estimates for nearshore Black (Sebastes melanops), Blue (Sebastes mystinus), and Deacon Rockfish (Sebastes diaconus). We surveyed Seal Rock Reef near Newport, Oregon, and compared our results to population estimates derived from a mark–recapture study conducted at the same reef. We compared fish density estimates from video deployments to those calculated from applying published target strength to length regression models to our acoustics data. Densities derived from the acoustics, using a generalized physoclist target strength to length model, were significantly different from densities derived from video; conversely, a rockfish-specific target strength to length model generated densities that were not statistically different from video densities. To assess whether, and how, fish behaviour was influenced by the presence of an underwater camera, we deployed our camera under the acoustic transducer. No statistical difference was observed in the acoustic density of fish before, during, or after camera deployment. Our work suggests that combining acoustic and stereo video data provided a similar population estimate to historic survey results, but an accurate acoustic density estimate was dependent on using the proper acoustic target–strength model. We contend that combining camera data with hydroacoustic data is effective for surveying rockfish in untrawlable habitats.

Improving the Accuracy of Krill Target Strength Using a Shape Catalog

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.

Echo-counting in the mesopelagic zone using a broadband split-beam acoustic scattering system

The ocean mesopelagic zone (200–1000 m depths) is a largely unexplored and remote ecosystem. Recent studies have suggested that the biomass of mesopelagic fish may be up to ten times larger than previously reported, more that the rest of the ocean combined. Interpretation of shipboard measurements of mesopelagic scattering layers in terms of abundance and biomass is complicated by large sampling volumes, complex assemblages of organisms, and highly uncertain target strength models. To address some of these challenges, Deep-See, a towed sensor platform carrying multiple broadband acoustic systems, has been developed, allowing real-time measurements of target strength and abundance. The focus here is on a new broadband split-beam echosounder (5.5 kHz–18.5 kHz) deployed in summer 2018 and 2019 off the New England shelf. This system is used to estimate the density of mesopelagic organisms using echo-counting and echo-statisticsapproaches. The parameters selected for echo-counting are discussed in order to achieve accurate estimates of abundance and also to identify and track individual targets. The calibrated broadband spectra of individual targets are used to determine the number of organisms with resonance frequencies in the band. [Funding provided by the WHOI Ocean Twilight Zone Audacious Project and an NSF MRI grant.]

Tests of Acoustic Target Strength and Bubble Dissolution Models Using a Synthetic Bubble Generator

AbstractTo test methods used for converting observations of acoustic backscatter to estimates of the volume and transport of free gas escaping the seabed, a bubble generator has been constructed and used at sea. The bubble generator creates individual bubbles of the sizes commonly associated with methane seeps, 1–5-mm radii, which can be released at preplanned rates. The bubble generator was deployed off the coast of New Hampshire at a depth of 55 m, and acoustic backscatter between 16 and 24 kHz was collected from a shipboard echo sounder while transiting over the rising bubbles. Bubble sizes and compositions (either Ar or N2) were known at the source. A model for bubble evolution, accounting for changes in bubble size and composition due to hydrostatic pressure and gas diffusion across the gas–liquid boundary, was coupled with an acoustic target strength (TS) model to generate predictions of the acoustic backscatter from bubbles that had risen to different depths. These predictions were then compared with experimental observation. Good agreement between prediction and observation was found in most cases, with the exception of the largest (4 mm) gas bubbles at depths of 30 m or less. The exact cause of this bias is unknown, but may be due to incorrect assumptions in models for the bubble TS, rise velocity, or mass transfer rate.

ZooScatR-An r package for modelling the scattering properties of weak scattering targets using the distorted wave Born approximation.

A thorough understanding of the scattering characteristics of marine organisms is a prerequisite for robust quantitative fisheries acoustic data processing or interpretation. Target strength models, such as the distorted wave Born approximation (DWBA) can be used to improve the understanding of field recordings of weakly scattering targets. With acoustic methods now being used by a wide audience, allowing access to such models becomes a necessity. To ease access to the DWBA model, an r package (zooscatr) which includes a web application and the ability to parameterise the model either through the web application, text files, or pure scripting has been developed and is now freely available on Github.

Measuring the in situ tilt orientation of fish and zooplankton using stereo photogrammetric methods

AbstractAcoustic‐derived estimates of fish and zooplankton numerical and biomass density rely on knowledge of the sound scattered by an individual, known as target strength (TS). An important, but difficult to measure, factor in determining TS is the tilt orientation of animals in relation to the insonifying acoustic wave. Underwater stereo‐camera systems can provide in situ tilt measurements for fish and zooplankton. We describe a protocol to remove the effects of camera pitch and roll from tilt measurements using a low‐cost orientation sensor and a mathematical correction. A tank experiment indicated that tilt estimates made with corrected pitch and roll position data improved measurement accuracy as compared to those made with uncorrected position data. This experiment also provided empirical limits on the range of yaw values that allowed for accurate estimation of target tilt and length. We further tested this protocol using an in situ collection of krill stereo images collected in the Gulf of Alaska. Krill tilts calculated with pitch‐ and roll‐corrected position data were an average of 10° different than those collected with uncorrected position data. Our pitch‐ and roll‐correction improved the accuracy of stereo photogrammetric tilt estimation. Extending this method to appropriate spatial and temporal scales could allow for more accurate parametrization of TS models.

In situ acoustic target strength of anchovy kilka (Clupeonella engrauliformis) in the Caspian Sea (Iran)

Understanding the factors that influence the target strength (TS) of fish is important for calculating accurate estimates of fish biomass and abundance using acoustic-based stock assessment techniques. The purpose of this study was to derive a robust TS model for anchovy kilka (Clupeonella engrauliformis), an important commercial-targeted fish species, based on its biological and morphological characteristics (length, weight, female maturity status, condition factor, depth of fish occupancy), and season. In situ measurements of TS were collected in the southern Caspian Sea during winter (February-March) and spring (May) 1997 using a Simrad EK500 echosounder operating a 38-kHz split-beam transducer. The results showed high variability in the mean TS estimates of anchovy kilka, despite a relatively small range in the sizes of fish encountered across the study site (range in mean fish lengths=3.2cm). A multiple linear regression analysis showed that the mean TS of anchovy kilka was significantly (P<0.05) correlated with indices of female maturity status and depth of fish occupancy, but not size (P>0.05), suggesting that biological characteristics other than size were important drivers of this variation. Our estimated TS model for anchovy kilka conformed to the equation: TS=20logL−0.096(%MF)−6.04log(Z10+1)−65.9 when the percentage of maturing females (MF, %) and the depth of fish occupancy (Z, m) were included in the analysis. These results help to improve the accuracy of acoustically-derived estimates of anchovy kilka abundance and biomass during routine stock assessments, which is important for robust resource management strategies in the Caspian Sea region.

Target strengths of two abundant mesopelagic fish species.

Mesopelagic fish of the Myctophidae and Sternoptychidae families dominate the biomass of the oceanic deep scattering layers and, therefore, have important ecological roles within these ecosystems. Interest in the commercial exploitation of these fish is growing, so the development of techniques for estimating their abundance, distribution and, ultimately, sustainable exploitation are essential. The acoustic backscattering characteristics for two size classes of Maurolicus muelleri and Benthosema glaciale are reported here based on swimbladder morphology derived from digitized soft x-ray images, and empirical (in situ) measurements of target strength (TS) derived from an acoustic survey in a Norwegian Sea. A backscattering model based on a gas-filled prolate spheroid was used to predict the theoretical TS for both species across a frequency range between 0 and 250 kHz. Sensitivity analyses of the TS model to the modeling parameters indicate that TS is rather sensitive to the viscosity, swimbladder volume ratio, and tilt, which can result in substantial changes to the TS. Theoretical TS predictions close to the resonance frequency were in good agreement (±2 dB) with mean in situ TS derived from the areas acoustically surveyed that were spatially and temporally consistent with the trawl information for both species.

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

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

An assessment of the use of ocean gliders to undertake acoustic measurements of zooplankton: the distribution and density of Antarctic krill (Euphausia superba) in the Weddell Sea.

A calibrated 120 kHz single‐beam echo‐sounder was integrated into an ocean glider and deployed in the Weddell Sea, Southern Ocean. The glider was deployed for two short periods in January 2012, in separate survey boxes on the continental shelf to the east of the Antarctic Peninsula, to assess the distribution of Antarctic krill (Euphausia superba). During the glider missions, a research vessel undertook acoustic transects using a calibrated, hull‐mounted, multi‐frequency echo‐sounder. Net hauls were taken to validate acoustic targets and parameterize acoustic models. Krill targets were identified using a thresholded schools analysis technique (SHAPES), and acoustic data were converted to krill density using the stochastic distorted‐wave Born approximation (SDWBA) target strength model. A sensitivity analysis of glider pitch and roll indicated that, if not taken into account, glider orientation can impact density estimates by up to 8‐fold. Glider‐based, echo‐sounder—derived krill density profiles for the two survey boxes showed features coherent with ship‐borne measurements, with peak densities in both boxes around a depth of 60 m. Monte Carlo simulation of glider subsampling of ship‐borne data showed no significant difference from observed profiles. Simulated glider dives required at least an order of magnitude more time than the ship to similarly estimate the abundance of krill within the sample regions. These analyses highlight the need for suitable sampling strategies for glider‐based observations and are our first steps toward using autonomous underwater vehicles for ecosystem assessment and long‐term monitoring. With appropriate survey design, gliders can be used for estimating krill distribution and abundance.

Material properties of Pacific hake, Humboldt squid, and two species of myctophids in the California Current

We measured the material properties of Pacific hake (Merluccius productus), Humboldt squid (Dosidicus gigas), and two species of myctophids (Symbolophorus californiensis and Diaphus theta) collected from the California Current. Density contrast (g) was measured for pieces of hake and myctophid flesh, and the following Humboldt squid body parts: mantle, arms, tentacle, braincase, eyes, pen, and beak. Density contrast varied with fish species, as well as among squid body parts. Effects of animal length and environmental conditions on nekton density contrast were investigated. Sound speed contrast (h) was measured for hake and myctophid flesh, Humboldt squid mantle and braincase, and varied within and between nekton taxa. These material property measurements can be used to more accurately parameterize target strength models and increase the accuracy of nekton biomass from acoustic surveys.

Accuracy of the Kirchhoff-Approximation and Kirchhoff-Ray-Mode Fish Swimbladder Acoustic Scattering Models

The acoustic backscatter from pressure release prolate spheroids and a three-dimensional representation of a fish swimbladder (Chilean jack mackerel, Trachurus symmetricus murphyi) was calculated using four target strength models (Kirchhoff-approximation, Kirchhoff-ray-mode, finite element solution of the Helmholtz equation, and prolate-spheroid-modal-series). Smoothly varying errors were found in the Kirchhoff-approximation and Kirchhoff-ray-mode model results when compared to the other models, and provide objective criteria for constraining the use of the KA and KRM models. A generic correction technique is also proposed for the prolate spheroid estimates and tentatively tested on a jack mackerel swimbladder, resulting in improvements to the target strength estimates from the Kirchhoff-approximation and Kirchhoff-ray-mode models.

Simulations of multi-beam sonar echos from schooling individual fish in a quiet environment

A model is developed and demonstrated for simulating echosounder and sonar observations of fish schools with specified shapes and composed of individuals having specified target strengths and behaviors. The model emulates the performances of actual multi-frequency echosounders and multi-beam echosounders and sonars and generates synthetic echograms of fish schools that can be compared with real echograms. The model enables acoustic observations of large in situ fish schools to be evaluated in terms of individual and aggregated fish behaviors. It also facilitates analyses of the sensitivity of fish biomass estimates to different target strength models and their parameterizations. To demonstrate how this tool may facilitate objective interpretations of acoustically estimated fish biomass and behavior, simulated echograms of fish with different spatial and orientation distributions are compared with real echograms of herring collected with a multi-beam sonar aboard the research vessel "G.O. Sars." Results highlight the important effects of fish-backscatter directivity, particularly when sensing with small acoustic wavelengths relative to the fish length. Results also show that directivity is both a potential obstacle to estimating fish biomass accurately and a potential source of information about fish behavior.

A distorted wave Born approximation target strength model for Bering Sea euphausiids

Abstract Smith, J. N., Ressler, P. H., and Warren, J. D. 2013. A distorted wave Born approximation target strength model for Bering Sea euphausiids. – ICES Journal of Marine Science, 70:204–214. Acoustic surveys monitor euphausiid populations in the Bering Sea because of their importance as prey for walleye pollock and other organisms. Various scattering models exist to convert acoustic backscatter data to estimates of euphausiid numerical density or biomass, but a target strength (TS) model specific to Bering Sea euphausiids has not been available. This study parameterized a distorted wave Born approximation (DWBA) scattering model using physical (length and body shape) and material (density contrast, g, and sound speed contrast, h) properties measured from live euphausiids. All model parameters (length, shape, material properties, orientation) were evaluated for their effect on predicted TS. A polynomial function was used to describe animal shape and produced smaller TS estimates compared to a taper function, as is traditionally used in DWBA scattering models of euphausiids. Animal length was positively correlated with TS, but variations in other parameters (including material properties and orientation) also produced large changes in TS. Large differences in TS between estimates calculated using measured versus literature material property values caused large variations in acoustic estimates of euphausiid numerical densities (animals m−3) which emphasizes the importance of collecting site-specific g and h measurements when possible.

Utility of 18-kHz acoustic data for abundance estimation of Atlantic herring (Clupea harengus)

Abstract Saunders, R. A., O'Donnell, C., Korneliussen, R. J., Fässler, S. M. M., Clarke, M. W., Egan, A, and Reid, D. 2012. Utility of 18-kHz acoustic data for abundance estimation of Atlantic herring (Clupea harengus) – ICES Journal of Marine Science, 69: 1086–1098. Current acoustic survey protocols for Atlantic herring (Clupea harengus) abundance estimation are principally dependent upon 38-kHz backscatter data. This can constitute a substantial problem for robust stock assessment when 38-kHz data are compromised. Research vessels now typically collect multifrequency data during acoustic surveys, which could be used to remediate such situations. Here, we investigate the utility of using 18- and 120-kHz data for herring abundance estimation when the standard 38-kHz approach is not possible. Estimates of herring abundance/biomass in the Celtic Sea (2007–2010) were calculated at 18, 38, and 120 kHz using the standard 38-kHz target-strength (TS) model and geometrically equivalent TS models at 18 and 120 kHz. These estimates were compared to assess the level of coherence between the three frequencies, and 18-kHz-derived estimates were subsequently input into standard 38-kHz-based population models to evaluate the impact on the assessment. Results showed that estimates of herring abundance/biomass from 18 and 38 kHz acoustic integration varied by only 0.3–5.4%, and acoustically derived numbers-at-age estimates were not significantly (p &gt; 0.05) different from 1:1. Estimates at 120 kHz were also robust. Furthermore, 18-kHz-derived estimates did not significantly change the assessment model output, indicating that 18-kHz data can be used for herring stock assessment purposes.

Acoustic determination of the distribution of fish and krill across the Scotia Sea in spring 2006, summer 2008 and autumn 2009

Multifrequency acoustic backscatter data were examined from transects at eight stations across the Scotia Sea, from the South Orkneys to the north west of South Georgia. These transects were repeated in austral spring (November 2006), summer (December 2007/January 2008) and autumn (March 2009). A dB identification window ( S v120-38 ) identified two types of scatterers dominating the acoustic backscatter; a positive S v120-38 indicative of Antarctic krill ( Euphausia superba) and a negative S v120-38 indicative of mesopelagic fish with swim bladders, such as myctophids. Acoustic backscatter attributed to schooling myctophid fish was greater at stations in the north of the Scotia Sea across all seasons and found to increase significantly with increasing mean sea temperature and absolute dynamic height. A schools analysis identified more fish schools at the stations with high fish acoustic backscatter. The number of fish schools increased, were distributed further south, and occurred deeper from spring through to autumn. Krill density, calculated using a target strength model, was similar in spring (27.7 g m −2) and summer (27.2 g m −2), but lower (2.2 g m −2) in autumn 2009. An analysis of swarm characteristics showed that during the spring cruise the high mean krill density resulted from a small number of swarms containing a large number of small krill, particularly in the south of the survey region. In contrast in the summer cruise the high mean krill density resulted from a large number of swarms containing a low number of large krill. Finally, in autumn the low krill density resulted from a small number of swarms containing a small number of large krill. Krill swarms were located at a mean depth of 58 m in spring, 94 m in summer and 123 m in autumn and were generally further apart, deeper, contained fewer krill and smaller in the north of the survey region. Large-scale shifts in the latitudinal and vertical distribution of krill and schooling mesopelagic fish in the Scotia Sea were observed. This seasonal reorganisation in both horizontal and vertical distribution will alter the role of these prey items in the ecosystem.