Deformed Cylinder Model Research Articles

Target strength measurements of Nile perch ( Lates niloticus: Linnaeus, 1758) in Lake Victoria, East Africa

Results of estimation of target strength (TS) of physoclistous Nile perch using in situ TS measurements and deformed cylinder model (DCM) at 70 and 120 kHz are presented. The results of the two methods are compared with those from cage experiments conducted between 1999 and 2000. Cage experiments that assumed the 20 Log( L) relationship (TS = 20 Log(TL) − 66.55) provided a good fit only over the length range of 18–38 cm total length, but overestimated the length of large fish (>60 cm TL). The DCM which lead towards a 30 Log( L) provides more accurate conversion over the full length range of single targets. An in situ study that made no assumptions about the shape, structure or orientation of the fish provided a close fit of acoustically estimated length frequency distributions to net hauls with the equations: TS 70 = 32.11 Log ( TL ) − 86.97 at 70 kHz and TS 120 = 30.15 Log ( TL ) − 84.14 at 120 kHz It is suggested that using this approach can provide valid TS to length relationships for acoustic surveys on species for which little information exists and there is little opportunity for direct observation of behavioural aspects that are known to affect TS such as tilt angle and vertical migration.

Differences in swimbladder volume between Baltic and Norwegian spring-spawning herring: Consequences for mean target strength

Fish swimbladders, where present, contribute most to the scattering of sound by fish, as measured by the target strength (TS). The volumes of the swimbladders of two different European stocks of Atlantic herring were compared to consider the effect on estimates of TS. Swimbladder volumes of Baltic and Norwegian spring-spawning herring ( Clupea harengus) were measured, together with individual herring fat content. Swimbladder volumes were found to differ significantly between the two herring stocks. Baltic herring have a larger swimbladder volume, which is suggested to be associated with the fish's low fat content, which in turn may be linked to its specific energy budget and the low salinity of the Baltic Sea. A buoyancy model that considered the different salinity conditions and fat proportions was used to evaluate the observed differences in swimbladder volume. The swimbladder volume data were subsequently used to model the mean target strength as a function of depth and growth pattern. Backscattering of the swimbladder was modelled using the modal-series-based deformed-cylinder model (MSB-DCM), describing the swimbladder as a gas-filled, elongated prolate spheroid. The fish body component was modelled as a fluid-filled ellipsoid using the distorted wave Born approximation (DWBA). Modelling results support a different TS-to-size relationship for Baltic herring, with a stronger echo, due to the larger swimbladder. Depth- and length-dependent TS relationships based on the model results are suggested.

Target strength of the lanternfish, Stenobrachius leucopsarus (family Myctophidae), a fish without an airbladder, measured in the Bering Sea

Abstract This paper reports theoretical values of target strength (TS) for the lanternfish Stenobrachius leucopsarus, a fish without an airbladder, which dominates the Subarctic marine mesopelagic fish community. Two models for liquid-like slender bodies, the general prolate-spheroid model (PSM) and the deformed-cylinder model (DCM), were used to compute the TS of the fish relative to its orientation. The relative mass density g and the sound speed h in seawater were measured and used in both models. To confirm the appropriateness of the models, tethered experimental measurements were carried out at 38 kHz for five specimens. The value of g measured by the density-bottle method was very low (1.002–1.009) compared with that of marine fish in general. The value of h measured by the time-average approach was 1.032–1.039 at the water temperature at which S. leucopsarus is found. TS-fluctuation patterns against fish orientation (the TS pattern) estimated from the DCM and PSM were in good agreement in the area of their main lobes. Both models reproduced the main lobes of the measured TS patterns in near-horizontal orientation (<±20°), and they were considered to be effective in measuring the TS of S. leucopsarus in a horizontal (swimming) position. After these comparative experiments, we computed the TS of 57 fish (27.8–106.9 mm) at 38, 70, 120, and 200 kHz, using the DCM. A plot of body length (in log scale) against TS showed a non-linear relationship at all frequencies. S. leucopsarus had a very low TS (<−85 dB, TScm), suggesting that acoustic assessment would be highly sensitive, especially when the proportion of small fish is high (e.g. L/λ < 2), and an appropriate frequency should be considered that takes into account both the length composition and the depth of occurrence.

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

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

Target strength of mesopelagic lanternfishes (family Myctophidae) based on swimbladder morphology

Abstract This article reports theoretical values of target strength (TS) for mesopelagic lanternfishes based on morphological measurements of their swimbladders. Three species of lanternfishes, Diaphus theta (26.9–77.4 mm standard length (SL)), Symbolophorus californiensis (85.0–108.4 mm SL), and Notoscopelus japonicus (126.0–133.2 mm SL), were examined. After external morphological measurement of the fish body, a specialized “soft X-ray” imaging system was used to map the swimbladders and obtain their morphological parameters. The swimbladder was inflated in D. theta, uninflated in S. californiensis, and was absent in N. japonicus. For D. theta, the swimbladder length does not increase in proportion to the body length, suggesting that the contribution of the swimbladder to acoustic reflection is reduced with growth in this fish. Based on the morphological measurements, the theoretical TS of the fish at 38 kHz was calculated using the approximate deformed-cylinder model (DCM) and the general prolate-spheroid model (PSM). For all three species, the calculations showed about 3 dB difference between the TS indicated by the DCM and PSM. Given that the description of body shape is poor in PSM, the DCM results were adopted for fish without a swimbladder or an empty one. The intercept b20 in the standard formula TS = 20 log SL + b20 was −85.7 dB (DCM) for S. californiensis and −86.7 dB (DCM) for N. japonicus. On the other hand, the PSM model was adopted for D. theta since its swimbladder has too small an aspect ratio to apply the DCM. For D. theta, the relationship between SL and TS is best expressed by TS = 11.8 log SL − 63.5, which implies that its scattering cross-section is not proportional to the square of the body length.

Modelling the effect of swimbladder compression on the acoustic backscattering from herring at normal or near-normal dorsal incidences

Abstract Inaccuracy in herring target strength can be an important source of bias in the acoustic assessment of several important herring stocks. New acoustic data on herring target strength (Ona et al., 2001, submitted for publication; Ona, 2003) confirm previous suggestions and evidence on a possible reduction of the size of the herring swimbladder as a result of its compression with increasing water depth. Theoretical work for a better understanding of the acoustic scattering from herring over its entire depth distribution may therefore be essential for improving abundance estimation. This study supplements the analysis, conducted by Gorska and Ona (2003) for herring averaged-backscattering cross-section. The modal-based, deformed-cylinder model (MB-DCM) solutions, presented in that paper, are used. The sensitivity of the herring backscattering cross-section in case of normal or near-normal dorsal incidences is studied with respect to frequency, contraction factors of the swimbladder dimensions and some fish morphological parameters. The study is important for a better understanding of not only the backscattering by individual fish for the dorsal incidence, but also the depth- and frequency-dependencies of the mean-backscattering cross-section. The theoretical results have been applied in the interpretation of the actual measured target-strength data on adult herring.

Modelling the acoustic effect of swimbladder compression in herring

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

Diurnal changes in the acoustic-frequency characteristics of Japanese anchovy (Engraulis japonicus) post-larvae “shirasu” inferred from theoretical scattering models

Abstract Diurnal changes in the acoustic-frequency characteristics of “shirasu”, a post-larval stage of Japanese anchovy (Engraulis japonicus), were analysed based on theoretical scattering models. Since post-larval swimbladders contain gas at night but not during the day, the distorted-wave Born approximation (DWBA), based on the deformed-cylinder model, which assumes there is no gas in the swimbladder, was applied to daytime observations, and the fish-bladder resonance model, which assumes there is gas in the swimbladder, to those taken at night. The two sets of acoustic frequencies employed were 50 and 200 kHz, which are those used for shirasu commercial fishing, and 38 and 120 kHz, which are usually employed for acoustic surveys using a quantitative echosounder. During the daytime, differences in shirasu target strengths between the two frequencies were large, especially between 38 and 120 kHz, but at night there was little difference with frequency, suggesting that acoustic surveys for identifying and estimating the abundance of shirasu should be done during daytime, when differences occur between frequencies and the shirasu form dense schools.

Target strength of an oily deep-water fish, orange roughy (Hoplostethus atlanticus) II. Modeling

Orange roughy consist of approximately 18% lipids by weight, mostly as wax esters, and the lipids must be taken into account when modeling target strength. A deformed cylinder model incorporating the effect of temperature and pressure on sound speed through wax ester was used to scale experimental measurements of target strength to the temperatures and pressures where orange roughy live (approximately 6 degrees C, depths approximately 800-1300 m). The effect of decreasing temperature and increasing pressure is to increase the sound speed in orange roughy lipids. Modeling shows that the net effect of this is to reduce tilt-averaged target strength, 'TS', by approximately 2 dB. Adjusting experimental results to compensate for temperature and pressure effects gives a predicted 'TS' for a 35-cm orange roughy of -48.3 dB. Adjusting in situ estimates of orange roughy 'TS' for avoidance behavior [McClatchie et al., J. Acoust. Soc. Am. 106, 131-142 (1999)] suggests the correct 'TS' is approximately -47.5 dB, rather than -50 dB as previously reported [Kloser et al., ICES J. Mar. Sci. 54, 60-71 (1997)]. We conclude that experimental and in situ estimates now converge at a 'TS' of approximately -48 dB for a 35-cm fish.

Comparison of Ayu target strength estimates determined from tank measurement and calculations

Ayu, Plecoglossus altivelis, is one of the most popular freshwater fish which can be seen in many rivers in Japan. Especially in the North Basin of Lake Biwa, Ayu is the highest predator. It is important to know the spatial and temporal variations of Ayu biomass to understand the predator’s effect to the food chain in the lake. Acoustic survey has been conducted since 1995. There is little information about the target strength (TS) of Ayu. Since Ayu has an open swimbladder, it is necessary to measure TS before the swimbladder was deflated by the suspension method in a tank. Target strength of live Ayu was measured in a tank and a three-dimensional image of swimbladder was obtained by an x-ray computer tomography (CT) system. In addition, a side and a dorsal image of the swimbladder were obtained by a soft x-ray system. All measurements were done in live condition. Kirchhoff approximation was used for the calculation on the basis of the three-dimensional image of swimbladder. A deformed cylinder model and a prolate spheroid model were also used for the calculation on the basis of side and dorsal image of the swimbladder. Calculation and measurement are compared and discussed in this paper.

Modeling considerations for acoustic scattering by fluidlike elongated zooplankton: Model intercomparisons and recommendations for euphausiids and copepods

Predictions of acoustic scattering by fluidlike elongated zooplankton are made using the deformed cylinder model and Distorted Wave Born Approximation over a wide range of animal size, shape, material property inhomogeneities, and orientation, as well as acoustic frequency. Each shape represents a digitized form of the shape of a euphausiid or copepod at low-, medium-, and high-resolution sampling of the shape (e.g., at low resolution, a straight smooth finite cylinder is used and at high resolution, a rough tapered bent finite cylinder is used). The results are analyzed in order to determine the conditions under which the various degrees of complexity in modeling the scattering are required. For example, the predictions illustrate that for modeling the scattering for animals near broadside incidence, relatively simple models can be used (smooth homogeneous bodies). However, for animals off broadside, which can be the case for downward-looking echosounders insonifying euphausiids and cope- pods, roughness and sometimes inhomogeneities must be used to model the scattering accurately (roughness effects dominate scattering well off broadside). Comparisons are made with laboratory data involving euphausiids, shrimp, and copepods. [Work supported by ONR.]

Experimental verification of the deformed cylinder scattering model and its application for the calculation of backscattering from fish

The dorsal aspect target strength pattern of straight cylinders made of polystyrene (EPS) are measured and compared with the deformed cylinder scattering model (DCM) proposed by Ye [Zhen Ye, J. Acoust. Soc. Am. 102, 877–884 (1997)] in the range of the tilt angle from −50° to 50°. Both experiments and the theory agreed very well for the wide range of tilt angle. The deformed cylinder model is also applied to predict the target strength pattern of Ayu (Plecoglossus altivelis) which is a river fish with an open swimbladder. The dorsal target strength of Ayu is measured by keeping it in alive condition. Then its swimbladder’s shape is observed using a soft x-ray system and digitized. Digitized data is used for the calculation on the basis of the deformed cylinder model and also the shape of the swimbladder is reproduced carving a rectangular solid of EPS by hand. The measured TS pattern of alive Ayu, the physical EPS model of swimbladder, and the DCM calculation agreed very well. Expanded polystyrene is a useful material to verify the scattering theory of the gas model and can become an artificial fish model with a swimbladder. Scattering from a complicated shape as a fish is predictable using DCM.

Laboratory studies and theoretical modeling of bistatic scattering of fish

Bistatic scattering of fish, in particular forward scattering (scattering in the forward direction), has been identified as a potential means of detecting fish, supplementary to the traditional backscattering. A series of laboratory experiments has been conducted over the past 2 years to measure bistatic scattering characteristics of fish, including forward scattering strength and scattering patterns around the forward direction. It is observed that the forward scattering strength increases rapidly with acoustic frequency and is much stronger than the backscattering strength. The measured scattering patterns (at 120 and 200 kHz) suggest that the scattering is strongest in the forward direction when the incident wave is normal to the fish, and drops by 10–20 dB when the scattering angle deviates from the forward direction by a few degrees. In the latter case, the scattering strength is still much stronger than backscattering strength. These results are compared with a recently developed deformed cylinder model, and with a shelled cylinder model currently under development. [Work supported by the Science and Technology Agency of Japan, the Department of Fisheries and Oceans of Canada, and the National Central University of Taiwan.]

Consequence of swimbladder model choice and fish orientation to target strength of three New Zealand fish species

Information on fish orientation has lagged behind the development of models to estimate target strength from fish swimbladders, despite fish tilt angle being an important variable influencing target strength. Few studies compare models, because authors generally compare their own model results with experimental data. We contrast three models for estimating target strength from fish swimbladders, and compare the magnitude of the differences between models to the effects of fish tilt angle on target strength. The swimbladder models compared were the mapping method, a deformed cylinder model, and the equicylinder model. The deformed cylinder model should give average target strengths with accuracy between the “exact” mapping method solution and the “approximate” equicylinder solution. The effect on average target strength of having a 5° or a 15° standard deviation of tilt angles is far more significant than the choice of model used to estimate target strength. We estimate the first averaged target strengths for three New Zealand commercial fish species: southern blue whiting (Micromesistius australis Norman, 1937), red cod (Pseudophycis bachus Bloch and Schneider, 1801), and barracouta (Thyrsites atun Eupharasen, 1791). We suggest that the greatest gains in target strength accuracy may be made from acquiring better information on fish orientation, rather than from the development of more elaborate modelling methods.

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

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.

Sound scattering by zooplankton with arbitrary shape and orientation

One of the challenges in describing the sound scattering by finite elongated objects with applications to zooplankton lies in the complexity of their shape and physical properties. Several approximate models have been developed to date in this field including deformed cylinder model, which allows the cross section of the scatterer to vary along its elongated axis [T. K. Stanton, J. Acoust. Soc. Am. 86, 691–705 (1989)]. While valid for a wide range of material properties, this model is restricted to a normal or near normal incident wave and requires a target to have a high aspect ratio. In this study, taking advantage of the fact that many zooplankton can be approximated as weak scatterers, a distorted wave Born approximation can be applied to compute the backscattering from such scatterers. Different from the conventional Born approximation, a substitution of the internal field by the incident field with a modified phase is used to obtain the backscattered field expressed explicitly as a volume integral over the entire body of the scatterer. This model can be applied to more complicated shapes with arbitrary orientations and its validity has been tested for various cases in which the exact solutions exist. Comparisons with data are made. [Work supported by ONR.]

Acoustic scattering models of zooplankton.

Quantitative analysis of echoes from marine organisms such as zooplankton can only be possible with the use of accurate acoustic scattering models. In order to describe the scattering of sound by organisms, their size with respect to acoustic wavelength, shape, orientation, and material properties must be taken into account. In this talk, zooplankton backscatter data collected from a variety of investigators in both the laboratory and field are presented. The data are compared with single realizations and averages of the scattering predicted by the deformed cylinder model [T. K. Stanton, J. Acoust. Soc. Am. 86, 691–705 (1989)] and indicate that the animals behave acoustically as weakly scattering fluid elongated objects. The accuracy of current predictions is discussed. [Work supported by ONR.]