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A Review of Target Strength Estimation Techniques
Abstract
In recent years, much work has been done to determine the target strength of marine organisms. This effort has been motivated by the extensive use of acoustic techniques to obtain population abundance estimates of fish stocks [1,2]. There has also been an increasing interest in the use of acoustic techniques to study and quantify crustacean and plankton populations. The two commonly used acoustic quantification techniques, echo counting and echo integration, both require a determination of the acoustic size of individual scatterers in the population [3,4].
... These expressions, originally derived by Ehrenberg (1972) [using Eq. (20)] for echo intensity with identical form, are given for echo magnitude (i.e., not intensity) in Ehrenberg et al. (1981) and are also described in reviews in Stanton and Clay (1986) and Ehrenberg (1989). While use of one form over the other [Eq. ...
When a beam emitted from an active monostatic sensor system sweeps across a volume, the echoes from scatterers present will fluctuate from ping to ping due to various interference phenomena and statistical processes. Observations of these fluctuations can be used, in combination with models, to infer properties of the scatterers such as numerical density. Modeling the fluctuations can also help predict system performance and associated uncertainties in expected echoes. This tutorial focuses on “physics-based statistics,” which is a predictive form of modeling the fluctuations. The modeling is based principally on the physics of the scattering by individual scatterers, addition of echoes from randomized multiple scatterers, system effects involving the beampattern and signal type, and signal theory including matched filter processing. Some consideration is also given to environment-specific effects such as the presence of boundaries and heterogeneities in the medium. Although the modeling was inspired by applications of sonar in the field of underwater acoustics, the material is presented in a general form, and involving only scalar fields. Therefore, it is broadly applicable to other areas such as medical ultrasound, non-destructive acoustic testing, in-air acoustics, as well as radar and lasers.
... When calibrated so that both receive channels produced the same output for a target on the axis of the transducer, targets close to the axis of the narrow beam could be corrected for the change in transducer sensitivity due to the shape of the beam. Following Ehrenberg (1989), the intensities received on the wide beam (I w ) and the narrow beam (I n ) are ...
... When calibrated so that both receive channels produced the same output for a target on the axis of the transducer, targets close to the axis of the narrow beam could be corrected for the change in transducer sensitivity due to the shape of the beam. Following Ehrenberg (1989), the intensities received on the wide beam (I w ) and the narrow beam (I n ) are ...
... A different technique, the split-beam technique was introduced to ocean acoustics soon after the dual-beam technology became reality [8], although its application to fisheries acoustics started somewhat later [26,27,32,56]. In contrast to a dual-beam system, a split-beam system uses not only the amplitude information, but also the phase information. ...
Application of sonar technology to fisheries acoustics has made significant advances over recent decades. The echo-sounder systems evolved from the simple analog single-beam and single-frequency systems to more sophisticated digital multi-beam and multi-frequency systems. In this paper, a brief review of major technological advances in fisheries acoustics is given, as well as examples of their applications.
... Acoustic surveys measure the amount of scattered sound in the water column, so we must know the target strength (TS) of individual fish to convert the measured backscattered energy into estimates of fish biomass. TS has been measured directly with split-or dual-beam echosounders (in situ measurement) for many fish species (Ehrenberg, 1983; Foote, 1991). S. leucopsarus, however, is difficult to detect as a single target in the field, because it occurs in deep water (200e400 m) and does not have a gas-filled swimbladder that can provide high acoustic reflection (Taylor, 1968; Yasuma, 2004 ); in situ TS measurements are therefore not available. ...
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.
... We used a threshold value of –65 dB during echo postprocessing for data used in the echo-squared integration analyses (Thorne 1983). Single target analysis using split-beam data (Ehrenberg 1983 ) used a threshold of –55 dB. Integrated values were identified for 5 m vertical intervals and horizontal intervals of 20 min. ...
The distribution of fishes is influenced by a host of physico-chemical and biological variables, including temperature and oxygen, prey abundance, feeding or assimilation rates, and predation risk. We used hydroacoustics and midwater trawls to measure the vertical distribution of pelagic fishes during a series of research cruises on Lake Superior's western arm in 2001 and 2004. Our objective was to assess vertical structuring in the fish assemblage over varying light levels. We observed variability in vertical structuring of both ciscoes (Coregonus spp.) and their primary predator, the siscowet (Salvelinus namaycush siscowet). Our results indicate that deepwater predators and prey migrate extensively over a diel cycle. This migration pattern is most consistent with changes in the distribution of prey resources for siscowet and diel variability in predation risk controlled by changing light levels for ciscoes. The magnitude of vertical migration in ciscoes increased with higher abundance of siscowets, supporting predation risk as a driver of cisco distribution. This study describes the extent of vertical migration in each group of fish, provides a statistical description of the pattern, and discusses the implications for trophic interactions in the Lake Superior food web.
... Echo-integration, the most important acoustic method for fisheries resource surveys, has been used widely and effectively (Johannesson and Mitson, 1983). In recent quantitative echo-sounders the dual-beam or split-beam method has been adopted to facilitate accurate observation of in situ fish target strength (TS) (Ehrenberg, 1983). The dual-beam method utilises coaxial narrow and wide beams. ...
The echo integration method has been widely and effectively used for acoustic surveys of fisheries resources. In recent quantitative
echo-sounders the dual-beam or split-beam method has been adopted, which improves the accuracy of in situ target strength (TS) measurement. The dual-beam method utilises a coaxial narrow and wide-beam pair. In ordinary dual-beam
systems, echo integration is performed only for the narrow-beam signal, not for the wide-beam signal. If echo integration
is also performed for the wide-beam signal, more information and better accuracy and precision of the acoustic survey of fish
abundance can be gained. Comparing the integrator outputs of both beams, an index of avoidance behaviour of fish towards the
surveying vessel can be compiled and unreliable measurements caused by, for example, noise contamination and failure in bottom
detection can be found. Wide-beam integration is of better use when distributions of fish are shallow and sparse, because
the sampling volume is greater. Furthermore, the error caused by transducer motion in bad weather conditions is smaller for
the wide beam. By combining both beam echoes, the volume backscattering strength within individual fish schools can be measured precisely. The method is discussed theoretically. Experimental investigations were carried out
on walleye pollock (Theragra chalcogramma (Pallas)), in the Bering Sea by our quantitative echo-sounding system, which provides independent echo integration results
for both narrow and wide beams.
... This uncertainty in the target strength needed to be addressed to obtain an absolute measure of orange roughy biomass: a 6 dB difference in TS translates to a four-fold difference in absolute biomass estimates. The direct in situ method is reported to be the best for obtaining species' TS, using either dual or split-beam technology (Ehrenberg, 1983 ). These methods compensate for the fish's position within the acoustic beam whilst the fish is in its natural environment. ...
In situ target strength measurements of fish at 600 to 1200 m depth were made around a spawning aggregation of orange roughy off
the east coast of Tasmania in 1992. The target strength data showed many modes, none of which could be definitely and uniquely
attributed to orange roughy, partly because the orange roughy avoided the towed body housing the acoustics. Dominant modes
at −50 and −55 dB were attributed to myctophid fishes with standard length modes at 8.2 and 5.3 cm; these fish have gas-filled
swimbladders and were undisturbed by the towed body. Small modes at −44 dB and −31 dB were attributed to the macrourid Coryphaenoides subserrulatus and the morid Halagyreus johnsonii, respectively. The swimbladder of H. johnsonii is gas-filled, while that of C. subserrulatus contains a gas-filled spongy tissue matrix. No evidence was found of a separate mode at −36 or −41.3 dB, the previously reported
target strength values of orange roughy. Modelling and tethered experiments on orange roughy suggested the target strength
range for a 35 cm standard length fish was −47.2 to −53 dB. The modelling indicated values at the higher end of the range;
measurements taken at depth of a tethered fish indicated the lower end. The dominant mode in the in situ data at −50 dB (which ranges from about −48 to −52 dB) was probably associated with orange roughy as well as myctophids.
We concluded that the in situ target strength for a 35 cm standard length orange roughy is between −48 and −52 dB. Such a low target strength (compared
to other species from the same depth that have gas-filled swimbladders) makes acoustic assessment techniques using echo integration
very sensitive to the number of fish with gas-filled swimbladders.
... Both transducers were narrow beam and circularly symmetrical with 7 and 9 between half-power points, respectively. The pulse length was 0.3 ms, implying that krill had to be separated by at least 0.22 m in range to be delineated as single targets (Ehrenberg, 1989). ...
An area of high krill (Euphausia superba Dana) density was continuously monitored with down-looking and side-looking sonars over a period of 24 h. Measurements of
volume backscattering strength were used to describe the density of krill with depth and the vertical movement of krill over
time. In situ measurements were made of dorsal aspect target strength (TS) and, as krill moved into the near-surface layer (0–15 m), in situ measurements were made of lateral-aspect TS. The probability density function (PDF) of TS measurements made with the down-looking
transducer had a mode at approximately −73 dB. The PDF of TS measurements made with the side-looking transducer was broader
with a mode at approximately −67 dB. Sampled krill had a bi-modal length distribution with a major mode at 44 mm and a minor
mode at approximately 30 mm.
... The TS and beam pattern effects have to be separated by deconvolution (e.g., Clay 1983). Direct methods facilitate the removal of the beam pattern from single fish echoes but require dual-beam (Ehrenberg 1983) or split-beam (Foote et al. 1986) echosounders. Received November 11, 1994. ...
The accurate determination of target strength (TS) is essential in the conversion of area backscattering to fish biomass. We present the first detailed in situ TS measurements of Atlantic cod (Gadus morhua) and Atlantic mackerel (Scomber scombrus) in the Northwest Atlantic. Mean total lengths of cod and mackerel were 52 and 38 cm, respectively. The mean daytime and nighttime in situ TS distributions (split-beam method) of cod were -35.5 (b(20) = -69.8) and -35.2 decibels (dB) (b(20) = -69.5), respectively. There was a depth stratification of cod TS at night, as mean in situ TS was -31.7 dB between 140 and 220 m and -35.8 dB between 220 and 300 m. We argue that the effect of depth on swim-bladder inflation is primarily responsible for the lower cod TS near the bottom than higher up in the water column. Analysis of the same TS data in split-beam or single-beam mode yielded similar distributions, showing the reliability of the single-beam method. The mean in situ TS (single-beam method) of mackerel was -56.4 dB (b(20) = -88.0), with no day-night difference.
... A number of references to the earlier work on indirect target strength can be found in Ehrenberg (1989). Stepnowski (1998) and Hedgepeth (1994 included some of these earlier methods in simulation comparisons with those discussed below. ...
Acoustic surveys for determining fish abundance in shallow water present many challenges. One constraint to down-looking (or vertical) surveys is that the sampling volume is limited due to the short range and typically narrow beams of acoustic transducers. By utilizing more of the acoustic beam, into the side-lobes for example, the sampling volume may be increased. Although the echo energy contribution from the side-lobes is usually small, these echoes may influence target strength estimation and echo counting, and by extension affect abundance estimation either directly or indirectly. With the advent of wide dynamic range echosounders, it is possible to assess both small and large acoustic-size targets. In this case, it is possible to collect signals from large targets in the beam side-lobes, and therefore an algorithm to use side-lobe data is proposed. The smoothed Expectation Maximization method (EMS) is used to estimate densities and target strength of fish from only main-lobe and main-lobe with side-lobe observations in simulation, and suggestions are made for real data from surveys. Résumé − Utilisation des lobes latéraux d'un sonar monofaisceau, pour l'observation des échos de poissons et l'estimation de l'indice de réflexion et de l'abondance en petits fonds. Des campagnes acoustiques pour déterminer l'abondance des poissons en petits fonds présentent un certain nombre de défis. Une contrainte des campagnes d'observation vers le bas (verticale) est que le volume échantillonné est limité en raison de la portée réduite et de l'angle d'ouverture étroit des transducteurs acoustiques employés. En tirant avantage de l'existence des lobes latéraux, le volume échantillonné peut être augmenté. Bien que la contribution de l'énergie de l'écho provenant des lobes latéraux soit habituellement faible, elle peut influencer l'estimation de l'indice de réflexion et le comptage des cibles, et affecter l'estimation d'abondance soit directement soit indirectement. Avec l'arrivée d'échosondeurs à larges bandes, il est possible d'estimer à la fois des cibles de petite et grande taille acoustique. Il s'agit alors de collecter des signaux à partir de grandes cibles dans les lobes latéraux du faisceau, puis d'appliquer un algorithme spécifique à ces données. La méthode de lissage des estimations des moyennes par maximisation est utilisée pour estimer des densités et des indices de réflexion des poissons à partir des observations du lobe principal seul ou du lobe principal et du lobe latéral en simulation, et des suggestions d'application sur des données réelles de campagnes acoustiques sont faites.
... where e is the echo level, b=D 2 and D is the normalized beam directivity (Craig and Forbes, 1969; Ehrenberg, 1972 Ehrenberg, , 1983 Clay, 1983). The raw acoustic data were filtered in order to remove any echoes that were not from single fish. ...
The tilt-averaged target strength, , of southern blue whiting (Micromesistius australis) at 38 kHz was measured using swimbladder modelling and two in situ methods. Split beam estimates of for southern blue whiting were carefully screened for multiple echoes, but a few useful values of were obtained from the periphery of shoals even at 150–300 m depths. We compare of southern blue whiting derived from swimbladder modelling to split beam and deconvolution estimates of . The -length regression for M. australis has similar slope but lower intercept than the published regression for blue whiting Micromesistius poutassou. Predicted for M. australis are 2.9 to 4.3 dB lower than for M. poutassou of the same size. In contrast to the swimbladder results, split beam and deconvolution estimates of for M. australis were in line with the -length relationship for M. poutassou. The magnitude of the difference between modelling in situ results could arise from the assumed tilt distribution of fish used in the modelling calculations and the actual, but unknown
tilt distribution of the fish in situ. Acquiring information on the tilt distributions of southern blue whiting is essential to resolve the measured difference
between in situ and modelled estimates of target strength.
... Multiple frequencies discriminate zooplankton by size and can be effectively used to estimate a zooplankton size-abundance spectrum (Holliday et al., 1989). The development of dual beam and split-beam technology (Ehrenberg, 1983; Traynor and Ehrenberg, 1990; Ehrenberg and Torkelson, 1997) made it possible to estimate the target strength of individuals (for non-overlapping echoes) in situ and to determine swimming speed and direction of individuals through the acoustic beam. The split beam transducer has four quadrants, which combine to transmit, and two splitbeam pairs for receiving, which allows the measurement of the angular location of the target in two right angle planes. ...
The composition and structure of marine food webs and it's variation in space and time influence the quality and quantity of carbon exported from surface water, the concentration of inorganic nutrients, and the elemental ratios of particulate and dissolved pools in the ocean. Marine phytoplankton, microzooplankton and larger zooplankton important to the biological pump, range over several orders of magnitude in size, occur over widely varying vertical and horizontal scales, exhibit different behaviors (e.g., vertical migration), and may occur seasonally or episodically. Consequently, assessing the efficiency and variability of the biological pump remains a major challenge. Two different sensor systems used to assess the abundance and distribution of marine biota will be described. The first approach uses a split-beam digital echo sounder system and the second approach uses a high-speed digital line scan imaging system. The merits, limitations, and recent developments of these systems will be presented. Although hardware and software used to assess marine biota have improved significantly during the last few decades, researchers still face serious technical issues in relating the detected image or signal to a biologically useful measure. Recommendations from a recent workshop on sensor technology are discussed.
... Ehrenberg applied that general solution to this case of an axisymmetric beampattern to calculate the beampattern pdf (6) where is the value of the beampattern , is a random variable representing the spherical polar angle relative to the center of the axisymmetric beam, and is the pdf of the scatterer position . Formulas for calculating the beampattern pdf were first derived by Ehrenberg in [18], then further developed in [29] and [19] and reviewed in [5] and [30]. ...
When a sonar beam sweeps across a field of scatterers and insonifies a different set of scatterers in each ping, the echoes can fluctuate significantly from ping to ping. For a homogeneous spatial distribution of scatterers in which there are a large number of scatterers in each sonar resolution cell, the echoes tend to be Rayleigh distributed. However, the echoes can be strongly nonRayleigh when there is a small number of scatterers in the sonar resolution cell-a condition that can occur with a narrow beam sonar. For scatterers randomly distributed in the sonar beam, the corresponding random weighting associated with the beampattern also significantly contributes toward a non-Rayleigh distribution when the number of scatterers is small. In this paper, a general formulation for echo statistics is developed by combining equations derived by Ehrenberg etal. [Proc. Conf. Eng. Ocean Environ., vol. 1, pp. 61-64,1972 and J. Acoust. Soc. Amer., vol. 69, pp. 955-962, 1981] and Barakat [Optica Acta, vol. 21, pp. 903-921,1974] to account for a directional sonar beam involving an arbitrary finite number of scatterers, each with an arbitrary echo probability density function (pdf), each randomly located in the beam, and each correspondingly randomly weighted by the beampattern. Theoretical predictions are made, along with numerical simulations for validation, for a range of cases including: 1) a range of number of scatterers each randomly located in the beam and 2) several different echo pdfs of the scatterers. Here, a "single scatterer" could be a patch of scatterers whose overall dimensions are much smaller than the resolution cell of the sonar beam and multiple scatterers could be multiple small patches. Although the application is intended for volumetric patches, the formulation could be applied to areal patches under appropriate conditions. The formulation applies directly to the geometry of the short-range direct-path sonar in which there are no reflections due to boundar-
-
ies such as the seafloor and sea surface.
... With suf®cient number of echoes from ®sh populations, it is also possible to correct for the effect of the location within the beam using deconvolution techniques. Several algorithms have been described to do this (Craig and Forbes, 1969; Ehrenberg, 1983; Clay, 1983; Robinson, 1983; Hedgpeth et al., 1999). A widely used single-beam algorithm is a modi®cation of the original Craig±Forbes method implemented in the HADAS acoustic analysis program (Lindem, 1983); a software used primarily in lakes and coastal areas. ...
We compared results from analysis of split-beam and single-beam acoustic surveys using two data sets, one from the Baltic Sea (both systems at 70 kHz) and one from Lake Erie (70 kHz single beam, 70 and 120 kHz split beam). First, we show that there is a bias towards smaller targets associated with implementation of a commonly used algorithm for deconvolving target strength distributions from single beam data (modified Craig–Forbes). This bias is 0.8 dB for a circular transducer. Differences in water column fish density estimates were not large although the single beam analysis consistently resulted in lower fish densities (85–95% of split beam densities). Target strength distributions obtained by split and single beam methods were very similar for the Baltic Sea data resulting in almost identical overall average TS. This was also true for epilimnetic targets strength distributions in Lake Erie. However, hypolimnetic target strength distributions still yielded smaller target strengths with single than with the two split beam units.The split beam analysis yielded target strength distributions with a wider dynamic range and more detail than the single beam analysis. Nevertheless, the traditional single beam analysis gave results for larger targets (over −56 dB) that were comparable to the information gained from the split beam units.
... As a complement to measurements of volume backscattering strength, multi-beam and multi-frequency systems have been developed to aid in positioning animals in the acoustic beam, making behavioral observations, and determining size distributions. Dual beam and split beam systems can be used to measure the backscattering cross-sections of individual animals (Ehrenberg, 1974(Ehrenberg, , 1979(Ehrenberg, , 1989, providing that single targets can be resolved in the sample volume. Reuss and Jaffe (1992) and McGehee and Jaffe (1993) described a three-dimensional (3D) acoustic imaging system that can resolve backscattering into a 3D volumetric matrix, providing a means to delineate individual zooplankters and to unobtrusively observe their behavior. ...
Acoustic sampling has been used to investigate the ecology of Antarctic krill (Euphausia superba) and to provide information on dispersion and abundance necessary to manage their harvest. Population estimates based on multi-ship acoustic surveys have been used to set catch limits. More localized acoustic surveys have been conducted to study the response of land-breeding krill predators to local variations in their food supply. These and future surveys may result in additional controls on the fishery. In this context, the use of acoustics to survey euphausiids is reviewed and major sources of uncertainty are discussed. These issues are organized as they pertain to the two broad steps of acoustic surveys: (1) estimating the volumetric density of krill (measurement uncertainty) and (2) mapping krill distribution and estimating abundance (sampling uncertainty).
... All the ME70 echosounder beams can be configured in the splitbeam mode. The split-beam configuration is mainly used to compensate for transducer directivity in target-strength measurements (Ehrenberg, 1983). However, phase information from split-beam measurements can also be used to detect the water-bottom interface , for example for seabed mapping where, as the incidence angle increases, bottom detection switches from amplitude-based to phase-based methods. ...
Bourguignon, S., Berger, L., Scalabrin, C., Fablet, R., and Mazauric, V. 2009. Methodological developments for improved bottom detection with the ME70 multibeam echosounder. – ICES Journal of Marine Science, 66: 1015–1022.
Multibeam echosounders and sonars are increasingly used in fisheries acoustics for abundance estimation. Because of reduced side-lobe levels in the beam-array pattern, the new Simrad ME70 multibeam echosounder installed on board Ifremer’s RV “Thalassa” has been designed to allow improved detection of fish close to the seabed. To achieve this objective, precise and unambiguous detection of the water-bottom interface is required, which raises the issue of bottom detection, especially in the outer beams. The bottom-detection method implemented in the ME70 is based on the amplitude of the reverberated echo. Such an approach is efficient for vertical beams, but less accurate for beams with higher incidence angles, typically 30°–40° for the beam configurations used on RV “Thalassa”, where the incidence angle, the beam opening, and the nature of the seabed contribute to weakening the backscattered signal. Therefore, the aim of this study was twofold. First, we proposed to improve the current bottom-detection method based on the amplitude of the echo. Thanks to the split-beam configuration being available for all beams of the ME70, we also proposed to use the phase information in the backscattered signals of the outer beams, as is more commonly done with multibeam systems dedicated to seabed mapping. Then, we set a Bayesian estimation framework that takes into account the spatial continuity between adjacent echoes, giving more robustness to the bottom estimation itself. Results using data collected at sea for various bottom types are presented here.
... Accurate estimates of fish target strength (TS) are required to calculate biomass using echo-integration methods (Simmonds and MacLennan, 2005 ). TS measurements of in situ fish are generally preferred over those obtained from ex situ fish (Ehrenberg, 1983; Kloser et al., 1997). However, reliable TS measurements of in situ fish can be difficult to obtain because measurements can be biased by multiple targets; information on the species, and their size and orientation, is generally uncertain; and fish may not behave naturally as a consequence of the presence of the acoustic measuring instrument (Koslow et al., 1995). ...
Ryan, T. E., Kloser, R. J., and Macaulay, G. J. 2009. Measurement and visual verification of fish target strength using an acoustic-optical system attached to a trawlnet. – ICES Journal of Marine Science, 66: 1238–1244.
It is difficult to make acoustic target-strength (TS) measurements of fish behaving naturally in deep-water habitats. The fish may avoid the acoustic instrumentation, and, if measured, there is uncertainty about their species and their orientation relative to the incident sound. To address these issues, a novel acoustic-optical system (AOS) has been developed, which combines a battery-powered, dual-frequency, split-beam acoustic system with a low-light video camera. The AOS attaches to the headline of a commercial deep-water demersal trawlnet that herds fish past the AOS and to the codend. This paper describes initial trials of the AOS to measure calibrated TS of New Zealand orange roughy, validated with video images. The fish species were visually identified, and their behaviour and orientation were approximated. The trawl catch provided associated samples for species identification and measurements of their length and other biological metrics. The combination of acoustics and optics in a net-mountable system constitutes a powerful sampling tool with broader applications in fishery research and ecosystem investigations.
... Ehrenberg applied that general solution to this case of an axisymmetric beampattern to calculate the beampattern pdf (6) where is the value of the beampattern , is a random variable representing the spherical polar angle relative to the center of the axisymmetric beam, and is the pdf of the scatterer position . Formulas for calculating the beampattern pdf were first derived by Ehrenberg in [18], then further developed in [29] and [19] and reviewed in [5] and [30]. ...
Zooplankton and micronekton typically comprise important components of the sound-scattering layers in oceanic environments. This fact was first recognized in the late 1940s and has since led to the development and refinement of several acoustical techniques for studying zooplankton and micronekton distributions. Over the last decade, oceanographers and acousticians have been developing techniques for extracting size information from acoustical data. However, only recently have investigators turned to a new, direct technique for estimating animal size employing high-frequency, dual-beam acoustics. Although the dual-beam technique has a proven track record in the field of fishery acoustics, its application to studies of zooplankton and micronekton ecology represents a dramatic, new direction in the field of bioacoustic oceanography. The duel-beam technique enables investigators to analyze the single echoes returning from individual animals. Analyses of data from single-beam systems are confounded by the interaction between an animal's actual target strenth and its position in the beam. A dual-beam system circumvents this problem.
A recently developed published approach to predict echo statistics is applied to clutter data that were collected with a midfrequency sonar and published in a separate independent study. This method explicitly accounts for the (finite) number of unresolved scatterers, the statistics associated with the arbitrary scattering properties of the individual scatterers [but assumed to have identical echo probability density functions (pdfs) in this application], and beampattern effects which significantly affect the echo statistics due to each scatterer being randomly located in the sonar beam. The data had been categorized according to whether they were associated with bottom structures, diffuse compact clutter, and compact nonstationary (moving) clutter. In this paper, the recently developed method is incorporated in a two-component mixed pdf (mixed with a Rayleigh distribution to account for the diffuse background) to model the statistics of the three classes of clutter. This is the first such application of the model which had principally been validated only numerically. The degree to which the data are non-Rayleigh (heavy tailed) is reasonably predicted by the model and the number of scatterers per resolution cell is inferred for each type of clutter.
This paper presents the results of TS equations (Acoustic Target Strength) in the dorsal aspect of two commercial marine fishes (mutton snapper, Lutjanus analis and bluestriped grunt, Haemulon sciurus) in the Gulf of Batabanó, Cuba. Measurements were executed in Puerto Esperanza (Pinar del Río, Cuba) and Xcalak (Yucatan Peninsula, Mexico) in places of shallow and clear waters with weak current. A Dual Beam Echosounder with digital transducer DT 5000 and 129 KHz frequency was used. Fishes were measured at 3 m deep. The equation obtained for Haemulon sciurus was TS(db) =21.9 log L(cm) - 69.2, r =0.75 and reference value calculated to make it comparable with other authors of b20= -66.38; while for Lutjanus analis it was TS(db) =21.7 log L(cm)-69.2, r = 0.81 and b20= -65.68. These equations can be used in the calculation of TS-length (dorsal aspect) widespread equation for tropical species in Caribbean areas.
Ehrenberg, J. E. and Torkelson, T. C. 1996. Application of dual-beam and split-beam target tracking in fisheries acoustics. - ICES Journal of Marine Science, 53: 329-334. Dual-beam and split-beam acoustical techniques were originally developed to provide direct in situ measurements of the target strength of individual fish and plankton. The amount of information that can be extracted from these acoustic systems can be significantly increased when the techniques are used in conjunction with ping-to-ping target tracking. For example, target tracking can be used to isolate multiple target-strength measurements from an individual fish or plankton. These measurements can then be averaged to provide a lower variance estimate of the target strength. The measurement of the angular location of the target provided by split-beam systems further enhances the usefulness of tracking. The angular data measured on subsequent pings can be used to resolve returns from single and multiple targets and estimate the direction and speed of the fish as they pass through the beam. Information on fish speed and direction is particularly useful for fixed location acoustic studies. This paper uses a combination of actual data, simulation results, and analysis to describe the various applications and expected performance of acoustic systems that combine dual-beam and split-beam techniques with target tracking.
The principles of the standard-target sonar-calibration method are reviewed. Protocols for calibrating scientific echo sounders, multibeam sonars, and a parametric sonar are outlined, and examples are presented. The range of frequencies presently spanned by the standard-target method is 1-3200 kHz. This includes both narrowband and continuously broadband systems as configured for measurement use in the field. Advantages of accuracy, operational convenience, and cost effectiveness are noted.
The area of active bioacoustics involves the use of sound to study distributions of fish and zooplankton in aquatic environments. There have been significant advances in this area over the past 30 years, covering many categories, spanning technology and modeling. These advances, as witnessed throughout my career, are reviewed. Issues with past and current approaches are discussed as well as projections into the future.
The effectiveness of a split-beam echosounder system to reject echoes from unresolvable scatterers, thereby improving the measurements of in situ target strengths (TS) of individuals, is dramatically enhanced by combining synchronized signals from two or more adjacent split-beam transducers of different frequencies. The accuracy and precision of the method was determined through simulations and controlled test tank experiments using multiple standard spheres and 38- and 120-kHz split-beam echosounders. By utilizing the angular positional information from one of the split-beam transducers, additional corresponding TS measurements were shown to be obtainable from a juxtaposed single-beam transducer. Both methods were utilized to extract in situ TS measurements of Antarctic scatterers simultaneously at 38, 120, and 200 kHz. The ultimate efficiency of the multiple-frequency method is shown to be limited by phase measurement precision, which in turn is limited by the scattering complexity of targets, the signal-to-noise ratio, and the receiver bandwidth. Imprecise phase measurements also result in significant beam-compensation uncertainty in split-beam measurements. Differences in multi-frequency TS measurements provided information about the identity of constituents in a mixed species assemblage. The taxa delineation method has potential, but is limited by compounding measurementuncertainties at the individual frequencies and sparse spectral sampling.
Acoustic surveys for determining fish abundance in shallow water present many challenges. One constraint to down-looking (or vertical) surveys is that the sampling volume is limited due to the short range and typically narrow beams of acoustic transducers. By utilizing more of the acoustic beam, into the side-lobes for example, the sampling volume may be increased. Although the echo energy contribution from the side-lobes is usually small, these echoes may influence target strength estimation and echo counting, and by extension affect abundance estimation either directly or indirectly. With the advent of wide dynamic range echosounders, it is possible to assess both small and large acoustic-size targets. In this case, it is possible to collect signals from large targets in the beam side-lobes, and therefore an algorithm to use side-lobe data is proposed. The smoothed Expectation Maximization method (EMS) is used to estimate densities and target strength of fish from only main-lobe and main-lobe with side-lobe observations in simulation, and suggestions are made for real data from surveys.
This paper presents the results of TS equations (Acoustic Target Strength) in the dorsal aspect of two commercial marine fishes (mutton snapper, Lutjanus analis and bluestriped grunt, Haemulon sciurus) in the Gulf of Batabanó, Cuba. Measurements were executed in Puerto Esperanza (Pinar del Río, Cuba) and Xcalak (Yucatan
A three-dimensional sonar imaging system, Fish TV (FTV), was developed to observe the three-dimensional trajectories of macrozooplankton in situ. Consisting of two arrays of eight transducers, the sonar uses one of the arrays as a projector and the other as a receiver. In this way, a combination of 8 × 8, or 64 beams, is constructed. Spatial resolution is approximately 2° by 2° and the range resolution is 3 cm. The sonar is capable of recording up to five images every second. The data is stored digitally in a three-dimensional matrix which can be postprocessed. Tests performed in the lab with both test targets and freshwater shrimp indicate that the system has the sensitivity and accuracy to track euphausiids in the ocean. The system was deployed at sea on an ROV (Remote Operational Vehicle) and the tracks of euphausiid sized objects were recorded.
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.
In situ indirect methods of fish target strength (TS) estimation are analyzed in terms of the inverse techniques recently applied to the problem in question. The solution of this problem requires finding the unknown probability density function (pdf) of fish target strength from acoustic echoes, which can be estimated by solving the integral equation, relating pdf's of echo variable, target strength, and beam pattern of the echosounder transducer. In the first part of the paper the review of existing indirect in situ TS-estimation methods is presented. The second part introduces the novel TS-estimation methods, viz.: Expectation, Maximization, and Smoothing (EMS), Windowed Singular Value Decomposition (WSVD), Regularization and Wavelet Decomposition, which are compared using simulations as well as actual data from acoustic surveys. The survey data, acquired by the dual-beam digital echosounder, were thoroughly analyzed by numerical algorithms and the target strength and acoustical backscattering length pdf's estimates were calculated from fish echoes received in the narrow beam channel of the echosounder. Simultaneously, the estimates obtained directly from the dual-beam system were used as a reference for comparison of the estimates calculated by the newly introduced inverse techniques. The TS estimates analyzed in the paper are superior to those obtained from deconvolution or other conventional techniques, as the newly introduced methods partly avoid the problem of ill-conditioned equations and matrix inversion.
Fish sonars have long been used to survey and monitor migratory
salmon in rivers and at sea. However, research has been lacking in the
development of algorithms to extract fish tracks from data collected in
a riverine or oceanic environment. Current fish trackers, based on a
pulse-to-pulse tracking method, only work well under ideal conditions
when targets are well separated and the signal-to-noise ratio is high.
Fisheries biologists often have to identify fish traces visually from
raw echograms. This approach is both labor-intensive and time-consuming,
limiting the usefulness of hydroacoustic techniques for fisheries
management. This paper presents a fish-tracking algorithm which sorts
randomly distributed echoes into coherent fish traces. Fish counts
obtained with the algorithm compare well with visual counts at two quite
different sites in the Fraser River watershed. The key features of the
algorithm are: (1) the linking mechanisms among sequential fish echoes
are range-dependent; (2) the growth echo for a developing track depends
not only on its space-time relation with the previous track echo (the
pulse-to-pulse statistics) but also on its relation to the entire track
being constructed; (3) there are a total of only five echo-association
criteria in the algorithm; (4) the simplicity of the algorithm structure
provides a convenient platform for implementing specific and
sophisticated tracking criteria to meet specific needs; and (5) the user
can fully control the performance of the algorithm by choosing values
for the 11 well-defined tracking parameters
Acoustic techniques have been used extensively over the last twenty-five years for collecting data for fisheries management and research. This paper describes the basic techniques used for acoustic assessment of fisheries resources. The echo integration technique used for estimating the density of biological populations and the in situ target strength measurement methods used for determining the acoustic size distribution of individual marine organisms are described. Some typical applications for the acoustic techniques are presented.
Some three years ago the Marine Laboratory commenced a study of the feasibility of making population assessments by echosounder. The species of prime interest was the herring of the northern North Sea. The study involved a review of work in progress elsewhere, a decision about the most suitable kind of equipment to use, and consideration of the statistical mathematics required for the interpretation of results. The choice of equipment, and of the resolution required, was of course influenced by the known facts about the distribution of herring. I t was felt that the counting and sizing of individual echoes would provide more information than an integration method, as it offered the prospect of classifying fish in accordance with their target strength (T decibels). To make this a practical method for routine surveys a degree of automation was required, so that the observational task aboard ship could be kept to reasonable proportions. Since the population density of herring can be at times very high (as much as 10 or 20 per cubic metre in daylight schools) it was decided to aim for the highest degree of resolution attainable within the available funds. Our previous experience suggested that an acoustic frequency of 400 kHz was about the optimum, offering a practical range of about 150 m, and a total beamwidth of about 13", using a transducer small enough to mount in a towed body of reasonable weight, around 80 kg (Fig. 1).
The amplitude of a sonarecho from a fish depends upon the species and size of the fish, acoustic wavelength, aspect, position of the fish in the sonar beam, range and backscattering cross section. We simplify the problem to a single species and size of fish, vertically downward echosounding, single aspect, and nonoverlapping echoes. After removal of attenuation due to range and absorption two random functions remain. The position of the fish in the sonar beam is random and the scattering cross section for each trail is random. We assume that the fish have a uniform density (number/m³) and calculate the probability density function (PDF) for insonification and reception. We assume that the PDF of the envelope of the echo (excluding the variability of insonification and reception) has a Rayleigh PDF. Assuming two PDF’s are independent, we calculate the PDF of the echo envelopes w E (e). w E (e) depends upon the beamwidth of the sonar and the mean backscattering cross section. The theoretical PDF has the same shape as the measured PDF of echoes from alewife in Lake Michigan. We use the fit of the PDF’s to estimate the backscattering cross section and fish density. This calibrates the echo‐integration processing system. A profile of the density of alewife in Lake Michigan is shown.
Subject Classification: [43]30.40; [43]80.40.
The ECOLOG dual-beam echo-counting and sizing system is used in both a laboratory and a field situation with moderate densities of demersal fish. Free swimming fish in a large tank give estimates of backscattering cross section ( sigma //b//s) with low variance. Application in field surveys confirms that dual-beam acoustic data provide information on both size composition and density distribution, which is different than single-beam estimates but similar to and at least as precise as that provided by trawl nets.
A set of empirical equations has been developed for use in determining the target strength or acoustic cross section of an individual fish at any insonified aspect as a function of fish size and insonifying frequency in the range 1?L/λ? 100, where L is fish length and λ is acoustic wavelength. The equations were developed by interpolating experimental data obtained by insonifying individual fish as they were rotated about one of their principal axes. It was found that acoustic cross section σ is proportional to slightly less than L 2 for each aspect, indicating that σ is approximately proportional to insonified area. Since σ is almost proportional to L 2, a modified set of empirical equations was developed with σ exactly proportional to L 2, thus eliminating the dependence of σ on frequency. The resulting errors are relatively minor and in some situations the modified equations lead to considerable simplifications which make their use quite convenient.
The Rician probability density function (PDF) accurately describes acoustic scattering by individual live fish. A fish scattering model is proposed which contains concentrated scattering components (representing the swim bladder) and distributed scattering components (representing the fish skeletal structure). The gross directional backscattering cross section is associated with the concentrated component. The ratio of these two components gamma , may serve as a means of identifying different fish species and fish behavior from the scattering PDFs. Monte Carlo simulations of a swimming fish using the acoustic fish model give Rician PDFs.
The purpose of this paper is to deconvolve the beam pattern effect from the amplitude distribution of sonar echoes from fish to determine the scattering processes at fish. The paper is a continuation of our acoustic methods to measure fish abundance. It demonstrates a direct procedure for determining the fish density (fish/m3), the probability density function (PDF) of scattering processes at fish, and the PDF of echoes. The procedure uses nonoverlapping echoes from a single transducersonar system. The integral equation for the echo PDF is in Clay and Medwin [A c o u s t i c a l O c e a n o g r a p h y (Wiley, New York, 1977), pp. 476–482]. It relates the PDFs of the sonar output w E (e), transducer beam w T (b), and scattering process at the fish w F (e). I use the transformations b=exp(−x) and e=e 0 exp(−y) to change the integral to the standard form of the convolution integral. The derivation of expressions of the convolution and deconvolution using z transformations follow directly. We use a ‘‘home‐style’’ microcomputer for computations. Tests of the deconvolution technique on fish echoPDFs for Lake Michigan alewife show the presence of the alewife (10–12 cm length) and an unidentified group of larger fish having two to three times the length of the alewife. Deconvolution of Lake Superior data for 1978 indicate the presence of smelt and an unknown group of larger fish. Deconvolution of 1979 Lake Superior data gives an echo PDF that can be attributed to smelt. In both lakes, fish density estimates are between 10− 4 and 10− 3 fish/m3.
A technique is described for indirectly determining the average backscattering cross section of individual fish from the measuredprobability distribution function for the amplitudes of the envelopes of single fish echoes. The technique is based on the assumption that the on‐axis echo envelope is Rayleigh distributed. The validity of the Rayleigh assumption was tested by making controlled measurements of the envelope amplitude statistics of single fish echoes for two acoustic frequencies and four different fish lengths. These tests showed that the echo envelope data most closely fit the Rayleigh model when the ratio of fish length to wavelength was large. The indirect measurement technique described has been used to analyze some single fish‐echo amplitude data for Bering Sea walleye pollock. These results are compared with some measurements made using a dual beam, direct, target strength measurement system. Expressions for the bias and variance of the indirect estimation technique are derived in an Appendix.
The probability density function (PDF) of the peaks of the envelopes of sonarecho from live fish were measured at beam aspect. The measurements were made at 220 kHz and in a waveguide. The fish was the common shiner (N o t r o p i s c o r n u t u s) and was about 120 mm (about 18 acoustic wavelengths) long. The PDF of the echoes was approximately Rayleigh when the fish was moving gently. The backscattering cross section equaled 4.2×10−5 m2. Transformation of the PDF’s to a target strength display in decibels displaced the maximum of the PDF to the target strength equaling 10 log10 (σbs/A 0)+3 dB where σbs is the mean backscattering cross section and A 0=1 m2. The target strengths of the common shiner (120 mm) and the mummechog (F u n d u l u s h e t e r o c l i t u s, 100 mm) were measured as a function of aspect angle. Comparison of the experimental measurements and Love’s empirical target strengths for any aspect showed that the measured target strengths at broadside aspect were about the same and the target strengths at other aspect angles were several decibels less than Love’s values. Linear arrays of point scatterers were chosen to match the gross aspect dependence of the target strengths of the fish. The lengths of the arrays were 6.5 mm for the common shiner and 16.5 mm for the mummechog. These lengths were less than the lengths of the corresponding fish’s swim bladders.
A high-frequency, dual-beam acoustical technique for directly estimating the size distributions within zooplankton assemblages has been developed. In combination with data from echo inte- gration, acoustical size distributions can be used to apportion zooplankton numerical density and biomass concentration into different size classes. Two major conclusions can be drawn from calibration experiments and preliminary field studies using the technique: the backscattering in- tensities from individual zooplankters increase proportionally with the cubes of their lengths; and the combination of dual-beam and echo-integration techniques has the potential to produce rapid, high-resolution, size-specific data on zooplankton distributions in the water column.
Measurements have been made at 30 kc/s of the target strength of dead cod, herring, plaice and perch. Four independent series of measurements have been made between 1955 and 1959.In three of the series, false Onazote swimbladders were placed in the body of the fish to simulate the acoustic properties of the normal swimbladder. The results are tabulated and presented in a graph, the reference level being the target strength of a sphere of 2 m radius. The results are discussed in relation to the frequencies which are most likely to be suitable for echo-sounding on fish targets when quantitative data are required.
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This paper describes the advantages of using a narrow- and a wide-beam acoustic transducer in systems for estimating fish abundance. In the technique considered, the acoustic pulse is transmitted with a narrow beam and the echo is received on both the narrow and wide beams. The signals received at the two transducers can be used to determine the acoustic scattering cross section of the fish. The mean value of the acoustic scattering cross section can be used to evaluate the scale factor needed by echo integrators to obtain an absolute abundance estimate. The outputs of the two transducers can also be used to control the sampling volume in an echo counting system.
Fisheries biologists using acoustic stock assessment systems need a signal processor that will provide an estimate of the fish target strength density function. The estimated density function can be used by the biologist to determine the fish size distribution within the surveyed population and to Convert the output of an echo integrator into a density estimate. A method for extracting the target strength density function from the acoustic pulses reflected from the fish is described in this paper. The results of a Monte Carlo simulation of the technique are presented.
Acoustic techniques are being used throughout the world for assessment of fish stocks. The most common acoustic assessment technique is to process the output of an echo sounder using a procedure called echo integration. The echo integration technique is based on the fact that the integrated intensity of the acoustic signal scattered off a fish population is proportional to the product of the mean acoustic scattering cross section of the individual fish in the population and the number of fish acoustically illuminated. The mean acoustic scattering cross section must be known to obtain an absolute estimate of stock size using this technique. A great deal of effort has been expended on the development of methods for measuring this parameter. This paper discusses two methods for indirectly extracting the mean scattering cross section from the empirical distribution of single fish echo amplitudes. The first method discussed obtains the estimate of mean scattering cross section by first estimating the single fish target strength probability density function. The second method assumes that the square root of the scattering cross section is Rayleigh distributed. With this assumption, the estimation problem becomes one of determining the single parameter in the Rayleigh distribution. The performance of the two estimation techniques is evaluated using a Monte Carlo simulation.
In recent years, acoustic techniques have been used extensively to monitor and quantify fish stocks remotely. One of the parameters that is often measured is the target strength of the individual fish in the population. To obtain the target strength of a fish from its acoustic echo, the effect of the transducer beam-pattern factor must be removed from the echo amplitude. This paper discusses a number of different approaches for directly estimating the beam-pattern factor for an individual fish target. The effects of noise and interfering signals on the various estimation techniques are investigated. Expressions are derived for the bias and variance in the estimates of the beam-pattern factor as a function of signal-to-noise ratio and angular location of the target. Results of a Monte Carlo simulation of the effect of interference on the various processing techniques are presented. Some of the problems encountered in implementing the various techniques are discussed.
Sonar echoes from unresolved features tend to interfere with each other. When a sonar is moved through an area where features of interest are not resolved, echoes will fluctuate from ping to ping due to the interferences. These fluctuations are the subject of this paper. We classify both fish and the rough seafloor by analyzing the nature of echo fluctuations. Similar statistical techniques apply to all of the problems. We use the techniques to determine, from echo fluctuations, parameters that describe the physical situation. Fish: Two conditions are low density and high density of fish (density is defined as number of fish per unit volume), a) In the low-density case, fish are resolved and their individual echoes are distinguishable from each other. From single fish there are interferences between echoes from the anatomical features causing fluctuations. Echoes from resolved fish also fluctuate because of beampattern effects. After deconvolution (or "removal") of beam-pattern effects, echo amplitudes from resolved fish fit a Rician probability density function (pdf). b) In the high-density case, the fish are not resolved and echoes from individual fish overlap. These echoes interfere and cause the echoes to fluctuate. The Rayleigh pdf describes the instantaneous echo amplitude from a cloud or school of fish. We process echoes by picking the peak amplitude in a finite-time gate and obtain an extremal pdf. We examine both regions of density (a) and (b) and present methods we have developed to determine, from the nature of the fluctuations, properties such as fish size frequency distribution and possibly gross anatomical features, fish density, and to count occasional large fish swimming in a cloud of plankton or near the sea surface. Seafloor: Interference of echoes from individual protuberances such as rocks, nodules, and ripples causes fluctuations. The Rice pdf describes echo amplitudes from the seafloor. We combine Eckart acoustic-scattering theory for a downward-looking sonar and Rice statistics. As a result we describe echo fluctuations in terms of rms roughness of the seafloor and correlation area (product of x and y correlation distances) of the seafloor. We use boomer data to estimate seafloor microrelief as well as to predict echo -
fluctuations for a variety of other sonars.
Manual of Methods for Fisheries Resource Survey and Appraisal
- St Forbes
- O Nakken
The application of multibeam acoustic techniques to marine resource assessment
- J E Ehrenberg
- J J Traynor
- V M Kaczynski
Ping-to-ping variations in target strength
- J J Dawson
- W A Karp
A selected bibliography of acoustics in fisheries research and related fields
- S C Venema
- SC Venema