Acoustic Target Strength Research Articles

수산 음향 기법을 이용한 연안 저서 어군의 시.공간 분포 및 안정성 조사

Hydroacoustic technique was used to analyze spatiotemporal stability and distribution of demersal fish aggregations in the coastal region to overcome some limitations of the existing methods such as net and diving. The survey was carried out in the Baekeum Bay on the south coast of Korea in January 2007. The bottom depth in the study site ranges from 7 to 25 m. In order to outline aggregations of demersal fish initial scanning using 200 kHz split-beam transducer was randomly conducted over the large area. Having detected fish aggregation in the specific region, intensive acoustic survey of irregular star pattern was carried out along 14 transects across the area in question. The results of the acoustic survey show that all demersal fish aggregations are concentrated about 5 m from sea bottom having a slight slope and remain steady with no spatial or temporal variations during acoustic survey. The hydroacoustic method used in this study offers a new approach to understand vertical and horizontal distribution, spatiotemporal stability, and biomass estimate of demersal fish aggregations in coastal regions. Additionally, the number of individual fish estimated from in situ acoustic target strength data can be used to understand the standing stock of demersal fish aggregation.

담수역 징거미 새우(Macrobrachium koreana)의 음향 반사 강도 특성

Assessment and management of fisheries abundance in fresh water like a river or a lake is very important to maintain fisheries itself as well as tourist industry even if their scale is not much large. The species for catch in fresh water are mainly a mandarin fish, a carp, an eel, and others. Because oriental river prawn is a main prey of these species and the change in its abundance is directly related to their abundance change in fresh water, information on the abundance and distribution of the species are necessary. Hydroacoustic survey is known to one of the efficient method among several methodology. Information on acoustic target strength is key parameter to estimate abundance for acoustic survey. In this study, measurements on oriental river prawn, Macrobrachium koreana, were conducted for two high frequencies(200kHz and 420kHz) with tilt angle using automatic rotating system. The results of acoustic target strength obtained from the experiment were compared with those of acoustic scattering model, Distorted Wave Born Approximation(DWBA) model. For 200kHz, the result of acoustic target strength experiments was expressed in terms of the averaged target strength dependence on the body langth(BL, cm) as a following relationship; >

Reduction of harbour porpoise ( Phocoena phocoena) bycatch by iron-oxide gillnets

Reduction of harbour porpoise bycatch by use of high-density iron-oxide (IO) gillnets was tested in sea trials in the Danish North Sea bottom set gillnet fishery in September–October 2000. The trials were conducted as a controlled experiment with conventional gillnets as the control group. Eight porpoises were caught in the control nets and none in the IO nets, a highly significant reduction ( P < 0.01). Of the four fish species analysed only catch rates of cod ( Gadus morhua) were significantly ( P < 0.01) different between the two net types, with CPUE in the IO nets being ca. 70% of the CPUE in the control nets. Subsequent investigations in seawater tanks revealed that the difference in acoustic target strength of the two net types was not significant and that the nets behaved similarly under various water flow conditions. Based on laboratory tests of twine samples and analyses of catch composition we conclude that it is the mechanical properties of the IO nets, primarily the measured increase in stiffness, that are the main reasons for the differences in catch rates for cod and for porpoises between IO and conventional nets.

Probing the deep: Acoustic characterization of the Mid-Atlantic Ridge ecosystem

Implementation of the ecosystem approach to resource management potentially increases the use of acoustics during assessment surveys. A Census of Marine Life survey on the mid-Atlantic ridge during summer 2004 was an opportunity to develop a practical approach. The Norwegian vessel, G. O. Sars, is an acoustically-quieted platform equipped with a five-frequency (18, 38, 70, 120, and 200 kHz) echosounder, acoustic Doppler current profilers (ADCPs), multibeam sonars, and a deep-towbody. Paucity of information on species composition limited the ability to conduct a traditional acoustic survey. Acoustic structure independent of biological sampling was quantified and then integrated acoustic density and target strength observations with biological community composition and length frequency data. Species were not assigned backscatter thresholds or water column regions at the onset of analysis. Near-real-time products monitored biomass distributions: daily echograms, target strengths, and frequency-differenced echograms. Persistent biological layers at different depths occurred in the water column. Layers or components migrated toward the surface during dark hours. Modes of target strength frequency distributions and trawl catch compositions differed among layers. The use of acoustic and net technologies can be integrated with near-real-time analytic results to quantitatively characterize pelagic ecosystems. [Work supported by ONR, IMR, and NOAA Fisheries.]

Density and sound speed contrasts of the Japanese common squid Todarodes pacificus and their influence on acoustic target strength

In this study, density and sound speed contrasts were measured for the Japanese common squid Todarodes pacificus. Target strength (TS) data derived from an acoustic scattering model based on measurements of these two parameters were compared with TS values based on acoustic measurement data to determine whether the measured parameters are reasonable values for theoretical scattering model. Density contrast (g) was measured from the displacement volume and wet weight, while sound speed contrast (h) was measured from the acoustic measurements of travel time (time-of-flight method). The Kirchhoff ray mode model, which represents the squid body as a set of fluid-filled cylinders, was used to calculate theoretical TS. Mean g- and h-values of the squid were 1.029 and 1.041, respectively. Comparison with previous data showed that g was similar, whereas h was relatively high. The difference in the TS between the theoretical model using measured parameters and the acoustic measurement was within 3 dB. Thus, the measured g- and h-values are acceptable for theoretical models of squid.

Hydroacoustic in situ target strength of smelt ( Osmerus eperlanus (L.))

Dependence of the in situ acoustic target strength (TS) of smelt ( Osmerus eperlanus (L.)) on the total fish length ( L) was estimated from TS measurements made with a split-beam 120 kHz echo sounder with reference to fish length–frequency distributions in trawl catches. The studies were conducted in the eutrophic and turbid Lake Hiidenvesi in southern Finland. Smelt dominated the catches-in-number with proportions at different sampling stations ranging from 91.8 to 100%. A TS–length equation of the form TS = alog L − b was fitted through the modes in the TS-distributions and in the respective fish length distributions. The estimated model (±parameter standard error) was TS = 23.4 (±2.24)log 10 L − 68.7 (±1.82). With a pre-defined slope of 20, the fitted equation was TS = 20 log 10 L − 65.9 (±0.23). The estimated day–night difference in TS–length relationship was not significant ( t-test, p = 0.37).

한국 연근해에서 어획된 주요 12어종의 75 kHz에 대한 음향 반사 강도의 체장 의존성

Acoustic target strength (TS) of 12 commercially important fish species caught in the Korean waters had been investigated and their results were presented. Laboratory measurements of target strength on 12 dominant fish species were carried out at a frequencies of 75 kHz by single beam method under the controlled condition of the water tank with the 241 samples of dead and live fishes. The target strength pattern on individual fish of each species was measured as a function of tilt angle, ranging from <TEX>$-45^{\circ}$</TEX> (head down aspect) to <TEX>$45^{\circ}$</TEX> (head up aspect) in <TEX>$0.2^{\circ}$</TEX> intervals, and the averaged target strength was estimated by assuming the tilt angle distribution as N (<TEX>$-5.0^{\circ}$</TEX>, <TEX>$^15.0{\circ}$</TEX>). The 75 to fish length relationship for each species was independently derived by a least - squares fitting procedure. Also, a linear regression analysis for all species was performed to reduce the data to a set of empirical equations showing the variation of target strength to fish length and fish species. An empirical model for fish target strength(TS, dB) averaged over the dorsal aspect of 158 fishes of 7 species and which spans the fish length(L, m) to wavelength(<TEX>${\lambda}$</TEX>, m) ratio between 6.2 and 21.3 was derived: TS: 27.03 Log(L)-7.7Log(<TEX>${\kanbda}$</TEX>)-17.21, (<TEX>$r^2$</TEX>=0.59).

수중표적에 대한 고주파수 음향산란 해석

A mono-static high frequency acoustic target strength analysis scheme was developed for underwater targets, based on the far-field Kirchhoff approximation. An adaptive triangular beam method and a concept of virtual surface were adopted for c onsidering the effect of hidden surfaces and multiple reflections of an underwater targe t, respectively. A test of a simple target showed that the suggested hidden surface removal scheme is valid. Then some numerical analyses, for several underwater targets, were carried out; (1) for several simple underwater targets, like sphere, square plate, cylinder, trihedral corner reflector, and (2) for a generic submarine model. The former was exactly coinc ident with the theoretical results including beam patterns versus azimuth angles, and the latter suggested that multiple reflections have to be considered to estimate more accurate target strength of underwater targets. ※Keywords: High frequency acoustic scattering(고주파수 음향산란), Underwater target(수중표 적), Sonar cross section(소나 단면적), Target strength(표적강도), Kirchhoff approximation(키르 코프 근사), Hidden surface removal(은면삭제), Multiple reflection effect(다중반사 효과)

Spatial variability of orange roughy around the Northwest Hills on the Chatham Rise, New Zealand

We used research trawl survey data to describe the small-scale distribution and spatial continuity of orange roughy catch rates around the Northwest Hills complex of seamount features on the Chatham Rise to the east of New Zealand. Analyses revealed three distinct spatial patterns. The larger scale spatial structure in the catch rates was defined by the contours running east–west along the Chatham Rise. Orange roughy were caught in a depth band between 850 and 1200 m , a depth distribution that is well known. Superimposed on this larger scale structure was a north–south trend in the highest catch rates at 180 ∘ longitude. This was created by higher catch rates on the seamounts. There were large differences in catch rates at small separation distances due to the seamount effect. However, not all the high catches were associated with seamounts, some being taken on the ‘flat’ areas around the hills. At distances away from the seamounts greater than the decorrelation length scale the spatial distributions of orange roughy catch rates were not random, and did not conform to a Poisson process used to characterise random distributions. Data visualisation of the spatial distributions of orange roughy and groups of bycatch revealed strong patchiness and spatial separation by depth of swimbladder fish, elasmobranchs and squid and oreos. This has important implications for acoustic surveys. Orange roughy overlapped most closely with swimbladder fish distributions, suggesting they may be obscured by species with higher acoustic target strengths. The observed separation of acoustically distinct fish groups by depth makes ground-truthing of acoustic backscatter easier. However, patchiness complicates prediction of where the acoustically distinct fish groups will occur. We conclude that extensive trawling provides a necessary complement to acoustic survey in order to estimate the biomass of orange roughy.

Low target strength fish in mixed species assemblages: the case of orange roughy

AbstractUse of acoustics to estimate biomass remains problematic for mixed species assemblages, and the problem is particularlydifficult where the main species of interest has low acoustic target strength, as is the case for orange roughy. Despite their size andrelative abundance in mixtures, orange roughy have low acoustic backscatter due to their oil-filled swimbladder. Trawl catchesand acoustic target strength—length relationships at 38kHz were combined to estimate the species-specific area backscatteringcoefficients from a mixture of species on the flat areas around the North West Hills on the Chatham Rise to the east of NewZealand. Orange roughy constituted a large proportion (¯ x =48%) by weight in trawl catches, but their contribution to the areabackscattering coefficient was very low (<1%). Robust cardinalfish (30.1%), three species of rattails (White rattail=17.4%,Notable rattail=13.8% and Serrulate rattail=9.7%) and Johnson’s cod (10.8%) made much larger contributions to the areabackscattering coefficient ( values in brackets ) despite their relatively low catch rates (totaling an average of 7.6% of the catch byweight). We calculated the effect of changing the mean density of orange roughy on the backscattering coefficient by simulatingorder of magnitude increases or decreases in orange roughy densities. Increasing the density of orange roughy by a factor of10 times would only increase the area backscattering coefficient from the fish community by 3%. We conclude that changesin orange roughy density of this order are not likely to be detectable in acoustic backscatter using echo integration data. Thisdoes not mean that orange roughy cannot be separated from mixtures, but to detect them it will be necessary to use single-targetdiscrimination techniques such as the phase difference- TS methods [Barr, R., Coombs, R.F., Macaulay, G., 2000. Can wediscriminate between different deepwater fishes using a standard acoustics target strength ping? In: Proceedings of the 2000Conference on International Society for Acoustic Remote Sensing, Sydney, Australia, pp. 77–80].© 2004 Elsevier B.V. All rights reserved.

Towards the acoustic estimation of jellyfish abundance

Acoustic target strengths (TSs) of the 2 most common large medusae, Chrysaora hysoscella and Aequorea aequorea, in the northern Benguela (off Namibia) have previously been estimated (at 18, 38 and 120 kHz) from acoustic data collected in conjunction with trawl samples, using the 'comparison method'. These TS values may have been biased because the method took no account of acoustic backscatter from mesozooplankton. Here we report our efforts to improve upon these estimates, and to determine TS additionally at 200 kHz, by conducting additional sampling for mesozooplankton and fish larvae, and accounting for their likely contribution to the total backscatter. Published sound scattering models were used to predict the acoustic backscatter due to the observed numerical densities of mesozooplankton and fish larvae (solving the forward problem). Mean volume backscattering due to jellyfish alone was then inferred by subtracting the model-predicted values from the observed water-column total associated with jellyfish net samples. Zooplankton-corrected echo intensity/jellyfish density data pairs were in close agreement with linear relationships deter- mined previously from uncorrected data. Small sample sizes precluded recalculation of TS, but non- parametric pair-wise tests failed to detect any significant differences between echo intensities for jel- lyfish densities observed in the present study and echo intensities predicted for those densities by density-intensity relationships arising from the previous study. Previous linear density-intensity relationships had y-axis intercepts greater than zero. On the assumption that the positive intercepts were due to backscatter from unsampled mesozooplankton, new TS relationships were calculated from downward-adjusted density-intensity relationships. New values agreed closely with TS esti- mates determined elsewhere using single-target echo detection techniques. Given that estimates of jellyfish TS appear robust, it should now be feasible to identify jellyfish acoustically at sea and to assess their abundance, even in the presence of mixed mesozooplankton assemblages.

The influence of tilt angle on the acoustic target strength of the Japanese common squid (Todarodes pacificus)

Abstract To measure the influence of changes in tilt angle on the acoustic target strength (TS) of the Japanese common squid (Todarodes pacificus), we conducted a series of experiments to estimate TS in relation to tilt angle and swimming angle. Swimming angle was measured in a seawater tank using two infrared, underwater cameras under dark conditions. Ex situ measurements of TS in relation to tilt angle on live specimens using a fishhook and cage method were then conducted at 38 and 120 kHz; mantle length (ML) ranged from 21 to 27 cm (mean 24.75 cm). For the more precise TS measurement with tilt angle, another set of ex situ TS measurements relative to tilt angle was made at 38 and 120 kHz on tethered, anesthetized specimens in seawater. The mean swimming angle was −17.7° (±12.7° s.d.). The mean TS varied from −48.6 to −44.6 dB and was relatively higher at 120 kHz than at 38 kHz, in the order of 0.7 and 2.5 dB. The empirical relationship between TS (dB) and ML (cm) is given by TS = 20 log10(ML) − 75.4 (r = 0.81) at 38 kHz or TS = 20 log10(ML) − 73.5 (r = 0.64) at 120 kHz. Based on the tethered method for the anesthetized squid, the mean standardized TS values (b20) were found to be highly correlated with the tilt angle, and the resultant fitted equations for b20 were expressed as: b20 = −73.3 + 0.48 × Θ + 0.0122 × Θ2 + 0.00016 × Θ3 for 38 kHz and b20 = −72.6 + 0.53 × Θ + 0.0134 × Θ2 + 0.00014 × Θ3 for 120 kHz, where Θ is the negative tilt angle in degrees. The mean TS based on the measurements using live squid was higher than that of tethered measurements, i.e., 2.6 dB at 38 kHz and 4.0 dB at 120 kHz. The higher mean TS in the ex situ measurements for the live squid can be explained by the influence of the low tilt angle on the overall TS data. The results can be used to understand the influence of tilt angle on the TS of Todarodes pacificus and thus improve the accuracy of biomass estimates.

Diurnal variation in acoustic densities: why do we see less in the dark?

Diurnal fluctuations in total integrated echo abundance and in vertical density profiles were examined using data from the Norwegian combined acoustic and bottom-trawl survey for demersal fish during winter in the Barents Sea. The total echo abundance was about 40%–50% higher at day than at night. An unknown amount of fish was lost close to the seabed in the acoustic dead zone, but the systematic changes in the near-bottom vertical density profiles did not indicate that migration in and out of the dead zone was the major reason for the large diurnal differences in echo abundance. A more plausible explanation could be that diurnal changes in fish behaviour affect the mean acoustic target strength. Based on the present study, we recommend that the time series of acoustic surveys should be re analysed, taking the diurnal bias into account. Any comparison of the fish densities indicated by trawl and acoustic surveys will suffer if this bias is not corrected. We believe that model development utilizing this type of information is crucial for future ecosystem-based monitoring.

In situ target strength of the Baltic Sea herring and sprat

Using single- and split-beam hydroacoustic equipment (70 and 38 kHz), and gillnet and trawl catches, we derived the relationship TS = 25.5 log10 L − 73.6 (r2=0.95) between acoustic target strength (TS) and fish length (L, cm) for Baltic Sea herring (Clupea harengus membras L.) and sprat (Sprattus sprattus balticus (Schneider)). Fixing the slope to 20, which is a standard practice in hydroacoustics, gave TS = 20 log10 L − 67.8 (r2=0.91). Normally, the fisheries agencies around the Baltic use a TS–length relationship that is based mainly on data from the North Sea and the intercept-value in this equation is 3.4 dB lower than that reported in this paper. This difference corresponds to an approximately twofold difference in assessed stock biomass.

Effect of orientation of euphausiids and copepods on acoustic target strength: Implications for measurements from down-looking and side-looking acoustic systems

Multifrequency acoustics is a potentially useful tool for zooplankton ecologists to rapidly map distributional patterns and determine taxonomic and size composition of scatterers. This is typically done with down-looking or side-looking acoustic systems. The ability to estimate taxonomic and size composition acoustically depends upon accurate models of scattering from zooplankton. In this study, distorted wave Born approximation (DWBA) models of individual zooplankton were used to illustrate that the frequency dependence of expected scattering is different for down-looking and side-looking acoustic systems due to the differences in orientation of scatters within the sampled volume. The results show that peaks and nulls occur at different frequencies for each system with maximum target strength differences of 30 dB. The models were used to predict volume scattering (SV) based on zooplankton collected from MOCNESS tows. Predicted SV was compared to SV measured by both a down-looking HTI acoustic system and a side-looking TAPS. The results showed that changes in orientation can have large effects on predicted total SV, particularly at higher frequencies, and demonstrate that choice of orientation parameters is important and different parameters should be used when comparing predicted SV with measured SV from down-looking or side-looking acoustic systems.

In situ target strength of the Baltic Sea herring and sprat

Abstract Using single- and split-beam hydroacoustic equipment (70 and 38 kHz), and gillnet and trawl catches, we derived the relationship TS = 25.5 log10 L − 73.6 (r2=0.95) between acoustic target strength (TS) and fish length (L, cm) for Baltic Sea herring (Clupea harengus membras L.) and sprat (Sprattus sprattus balticus (Schneider)). Fixing the slope to 20, which is a standard practice in hydroacoustics, gave TS = 20 log10 L − 67.8 (r2=0.91). Normally, the fisheries agencies around the Baltic use a TS–length relationship that is based mainly on data from the North Sea and the intercept-value in this equation is 3.4 dB lower than that reported in this paper. This difference corresponds to an approximately twofold difference in assessed stock biomass.

Changes of acoustic target strength during juvenile perch development

Dependence of acoustic size (target strength—TS in dB) on body length of juvenile perch ( Perca fluviatilis L.) was studied. Perch larvae were reared in a laboratory tank from spawning to the age of 89 days. Fish length and TS were measured during 3, 7, 15, 22, 45 and 89 days of life. TS was measured using a Simrad 500 EY split-beam echosounder with the frequency 120 kHz and a BioSonics Model 101 dual-beam echosounder with the frequency 420 kHz in down-looking, horizontal and up-looking directions. Range of fish body length 7–41 mm corresponded with a TS range from −69 to −53 dB. Differences between TS of different aspects were not found in young fish larvae. TS in ventral and dorsal aspects appears to grow faster than in the side aspect for fish 15–30 mm long. With perch larger than 40 mm, vertical and side aspects reach similar values. This paper provides regressions for the size identification of acoustically detected fish targets. The results indicate that the TS/length relationship of early life stages of perch is not too different from general fish TS/length relationship.

부세에 대한 음향반사강도의 체장 의존성

우리나라 주변해역에 서식하는 주요 어종의 체장식별에 필요한 음향반사강도의 data bank를 구축하기 위한 연구의 일환으로 대형음향수조에서 부세의 체장에 따른 음향반사강도를 측정하고, 그 반사강도의 체장 의존성 및 KRM 산란모텔에 의한 이론 반사강도와 측정치를 비교, 분석한 결과를 요약하면 다음과 같다. 1. 체장의 범위가 21.0-32.5 cm인, 총 13마리의 부세를 대상으로 구한 평균반사강도패턴에서 최대 반사강도는 tilt angle이 -13.35<TEX>$^{\circ}$</TEX> 인 입사방향에서 나타났고, 이 때의 반사강도는 -35.13 ㏈ 이었다. 2. 부세의 체장 (fork length, cm)과 평균반사강도의 사이에는 다음의 회귀직선식이 성립하였다. TS=23. 76log(L) -73.45(r=0.47) TS=20log(L) -67.35이 결과로부터 부세의 반사강도는 체장의 2.376승에 비례하는 경향을 나타내었고, 체장변환계수는 체장의 2승에 근사시켜 구한 값 보다 6.1 ㏈ 더 낮게 나타났다. 이들의 결과는 향후 체장어군탐지기의 체장변환계수의 값으로서 활용될 것으로 판단된다. 3. 평균체장은 25.59 cm인 총 13마리의 부세에 대한 반사강도의 빈도분포로부터 구한 반사강도는 -41.23 ㏈이었고, 이 값으로부터 구한 체장변환계수는 -69.72 ㏈이었다. 4. KRM model에 의한 부세의 이론반사강도는 L/λ 비가 15보다 작은 영역에서는 파동상의 변화를 나타내면서 증가하는 경향을 나타내었고, 15보다 큰 영역에서는 매우 완만하게 증가하는 경향을 나타내었다. 5. 부세의 평균반사강도의 측정치는 KRM 산란모델에 의한 예측치보다 전반적으로 낮은 경향을 나타내었고, 보다 정량적인 분석을 위해서는 광범위한 주파수 범위에 대한 실험이 필요하다고 판단된다. This paper was conducted as an attempt in order to construct the data bank of target strength for acoustic estimation of fish length in the coastal waters of Korea. The fish length dependence of acoustic target strength for 13 large yellow croakers (Pseudosciaena crocea) at 75 kHz was investigated and the prediction of the target strength by using the Kirchhoff-Ray Mode model (KRM model) was compared with target strength measurements. The results obtained are summarized as follows; 1. In the averaged target strength pattern for 13 large yellow croakers the maximum target strength was -35.13 dB at <TEX>$-13.35^{\circ}$</TEX> on a tilted angle. 2. The relationship between fork length(L, cm) and averaged target strength(TS, dB) was expressed as follows; TS=23. 76log (L) -73.45 (r=0.47) TS=20log(L) -67.35 From this result, the conversion coefficient was -73.45 dB and 6.1 dB lower than the coefficient -67.35 dB where the value of the slope of the regression equation is forced to be 20. 3. Averaged target strength and a length conversion coefficient derived from a target strength histogram for 13 large yellow croakers of mean length 25.59 cm were -41.23 dB, -69.72 dB, respectively. 4. In the range of <TEX>$$2;{\ll} L (fish length /{\lambda}(wave length);{\ll}40$$</TEX>, the prediction of the averaged target strength by the KRM model increased gradually with the increasing of <TEX>$L/{\lambda}$</TEX> and was lower than the measured target strength.

Quantification of a multibeam sonar for fisheries assessment applications.

The acoustic theory is developed for a multibeam fisheries-type sonar employing a circular arc of transducer elements. Specifically, numerical relations for transmit and receive beam patterns are derived and methodologies set forth for the derivation of appropriately scaled acoustic target strength and acoustic volume backscattering strength from an ideally performing multibeam device. Predicted and measured beam characteristics of a realizable multibeam sonar, a Kongsberg Simrad-Mesotech SM 2000, are compared. Practical techniques for the extraction of calibrated acoustic volume backscattering strength from real systems are advanced.

In Situ Measurements of the Acoustic Target Strength of Cape Horse Mackerel Trachurus Trachurus Capensis Off Namibia

The acoustic target strength (TS) of Cape horse mackerel Trachurus trachurus capensis was measured in situ at 38 kHz during two surveys over the Namibian continental shelf in 1998 and 1999 using a SIMRAD EK500 echosounder/ES38D submersible split-beam transducer. Scattered aggregations of horse mackerel 100–200 m deep were ensonified. The transducer was lowered to a depth of 85–140 m in order to resolve single targets at short ranges (5–50 m). Individual fish were tracked using specially developed software. Samples of ensonified fish were obtained using pelagic and demersal trawls; the former was fitted with a codend Multisampler for depth-specific sampling. Recorded TS estimates were low, producing b 20-values ranging from −77.5 to −74.9 dB (−76.0 dB ± 1.3), considerably lower than published estimates for horse mackerel (−73.4 dB < b 20 < −65.2 dB). An explanation for the weak acoustic backscattering may be swimbladder compression. Surface-projected b 20 values, which were computed using the depth of each target and the scattering area reduction rate previously found for herring (γ=−0.29), corresponded to −72.6 dB. This value is close to the TS constant of −72 dB currently applied for horse mackerel in Namibian and Angolan waters.