Target Strength Values Research Articles

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.

Prediction of hydroacoustic target strength of vendace ( Coregonus albula) from concurrent trawl catches

Despite frequent applications of hydroacoustic methods to estimate freshwater fish densities and behaviour, species-specific regressions to predict fish total length (TL) from target strength (TS) and vice versa are surprisingly rare in literature. Here, night trawl catches and trace track analyses of concurrent hydroacoustic surveys of a vendace stock in a deep lake during three sampling occasions over 2 years were combined. Frequency distributions of TL and TS revealed three modal lengths on each occasion and were used to calculate a linear regression (TS (dB) = 25.5 × log 10 TL (cm) − 70.9) which predicted higher TS values than the standard formula by [Love, R.H., 1971. Dorsal aspect of an individual fish. J. Acoust. Soc. Am. 49, 816–823] for vendace smaller than about 12 cm TL, but lower TS values for larger fish. In an alternative approach, an iterative estimate of the intercept of the TS–TL equation based on the main mode per frequency distribution was conducted and this also substantially differed from the general formula of Love. By correlating the cumulative TS distributions from the echo tracking and those calculated from the equations based on either three or one mode per sampling and the species-unspecific formula of Love, the three modes approach and the Love formula performed comparably well, whereas the one-mode approach did not reliably predict the TS distribution of the tracks. A re-calculation of a previous hydroacoustic survey from the same lake revealed that the newly derived vendace-specific formula predicted a lake-wide vendace biomass about 13.8% lower than one based on the formula of Love.

Lidar target-strength measurements on Northeast Atlantic mackerel (Scomber scombrus)

AbstractA linearly polarized green (532 nm) laser and a digital video camera were used to determine the reflectivity (R) and lidar (LIght Detection And Ranging) target strength (TS) of live mackerel by comparison with a standard calibration target. The measured reflectivity was 0.0141 ± 0.0005 when the receiver was copolarized with the laser and 0.0092 ± 0.0004 when the receiver was cross-polarized. The corresponding TS values were −42.66 ± 0.24 dB for the copolarized channel and −44.86 ± 0.23 dB for the cross-polarized channel. The depolarization ratio (depolarized return over total return) of 0.396 is very different from earlier measurements of sardine, suggesting that depolarization might be useful for species identification.

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 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.

Comparing two 38-kHz scientific echosounders

Abstract The EK500 has been the state-of-the-art scientific echosounder for surveying marine fish stocks for over a decade; the EK60 is its successor. Ensuring comparability in performance is vital during the transition from the EK500 to the EK60. To quantify the respective performances, each echosounder was calibrated in tandem by the standard-target method using the same 38-kHz, 12° beam width, split-beam transducer, with alternating pinging by means of an external triggering-and-switching system. The principal measurements comprised split-beam-determined angle and target strength, on-axis sensitivity, and directionality in the plane normal to the acoustic axis, as measured with a 60-mm-diameter copper sphere. Ambient noise, including volumetric reverberation, was also measured. Principal comparisons included those of the time-series and histograms of split-beam-determined target strength; respective alongship and athwartship angles as determined by the split-beam system; and as expected, difference in the split-beam-determined and experimental target-strength values in the plane normal to the acoustic axis. The mean absolute difference in off-axis angle values was also compared. While the performance of the two echosounders is generally similar, systematic differences exist. For the particular calibration measurements, the time variability in measurements of on-axis target strength was of the order of 1 dB for the EK500 and 2 dB for the EK60. The target-strength distribution for measurements made with the EK500 was normal, with standard deviation 0.2–0.3 dB, whereas for the EK60, the target-strength distribution was distinctly skewed and the standard deviation varied over 0.3–0.5 dB. Differences were found between the split-beam and physical-angle measurements. They were noticeably larger in the case of the EK60. Differences in performance between the two echosounders suggest refinements to the new system that will help realize its full potential in scientific work.

Patterns in tidal migration of fish in a Brazilian mangrove channel as revealed by a split-beam echosounder

A 200-kHz circular split-beam echosounder (BioSonics, DT6000) was placed in the center of a macrotidal mangrove channel in north Brazil to study the movement patterns of tidal migrating fish. Vertical beaming gave high signal-to-noise ratios (21 dB) during neap tides in the dry season 2002. Despite low Secchi depths, a diel change in the vertical target distribution was apparent when fish inhabited the water column only during the night flooding. Moreover, responses in vertical distribution occurred at dusk and dawn. The multispecies population simultaneously caught with a tidal trap consisted of juveniles and adults of small species, and juveniles of larger species (mean total length of fish: 14 cm), being reflected in target strength values ranging from −60 to −40 dB. Nekton organisms usually traveled with the tide. At low water fish concentrated in the subtidal channel sections, swam slowly and meandered in different directions. At first flood rise—the strongest ambient change—the entire fish population ‘rode the tide’ to immigrate into the intertidal zone, indicated by fast, linear, upstream tracks throughout the water column. At slack high water fish milled around (likely Cetengraulis edentulus and Anchovia clupeoides at night and epibenthic fish in daytime). When sampling the spatiotemporal distribution of fish in intertidal environments, the 3D spatial heterogeneity versus time should be considered. A net upstream longitudinal current together with a regular first flood rise likely promoted retention of fish in this mangrove nursery from one tide to the next. In well-mixed shallow-water environments with a dominant epibenthic fish population, horizontal and vertical beaming should be combined since deeper sections may serve as a refuge for the fish.

Single-target echo detections of jellyfish

Abstract Acoustic target-strength (TS) measurements are presented for tethered and free-swimming individual Chrysaora hysoscella (Scyphozoa) and Aequorea aequorea (Hydrozoa) medusae in Namibian waters. Tethered individual C. hysoscella (17–54 cm total umbrella diameter) and A. aequorea (19–28 cm total umbrella diameter) were ensonified at 38 kHz using a portable echosounder. Mean TS values for individual medusae at this frequency ranged from −67.3 to −52.8 dB for C. hysoscella and from −65.4 to −50.1 dB for A. aequorea. There was a positive relationship between medusa diameter and TS for both species. TS of individual medusae varied cyclically over time by about 15 dB, probably because of the periodic contraction of the medusae whilst swimming. C. hysoscella was parasitized by hyperid amphipods (maximum infestation >1800 parasites per medusa). A fluid-cylinder scattering model was used to determine the expected backscatter from the parasites, and it suggested that even at the highest observed level of infestation the jellyfish itself remained the major contributor to total backscatter at 38 kHz. Single-target echoes from targets identified by trawling as medusae were obtained from vessel-mounted echosounders at 18, 38, 120, and 200 kHz. Triangulation between echosounder beams to identify targets detected simultaneously at all four frequencies increased confidence that echoes were in fact from single targets. The 38-kHz TS values from free-swimming medusae corresponded with values obtained from tethered animals at the same frequency, providing strong evidence that the TS estimates were robust. TS values at all four frequencies (Chrysaora hysoscella mean umbrella diameter 41 cm, TS at 18 kHz = −60.0 dB, 38 kHz = −65.5 dB, 120 kHz = −68.0 dB, and 200 kHz = −70.5 dB. Aequorea aequorea mean inner-umbrella diameter 6.5 cm, TS at 18 kHz = −66.0 dB, 38 kHz = −65.5 dB, 120 kHz = −68.0 dB, and 200 kHz = −73 dB) were consistent with previously published data. Given these robust TS estimates, the possibility may now exist for multi-frequency identification and evaluation of these jellyfish species in some circumstances, and for the use of acoustic-survey techniques to estimate jellyfish abundance.

Simultaneous Sv and TS measurements on Young-of-the-Year (YOY) freshwater fish using three frequencies

Abstract In autumn, the fish population above the thermocline in Lake Annecy mainly comprises young perch (Perca fluviatilis) of the year. The fish are distributed in schools during the day and as scattered individual fish targets at night. Measurements of volume-backscattering strength and target strengths (TS) were carried out in a synchronous way using three echosounders operating at three frequencies (70, 120, and 129 kHz), with different characteristics (split beam, dual beam, beam shape, pulse length, etc.). Target-strength values show variability from one elementary sampling unit to another and from one device to another, but the mean global TS values are similar, independently of the frequency. The volume-backscattering strengths measured on precise schools by the three acoustic devices give significantly similar results. Acoustic measurements on YOY perch are completely independent of the frequency and the characteristics of the echosounders.

Studies on geometrical backscattering models of marine bodies

The target strength of marine bodies depends on two components—one of them is easy to measure and a good relationship can be established with the target strength value, while the other presents a higher variability that no method can help to reduce, which include the orientation of the fish and a range of environmental conditions and a set of biological facts. Studies on physical models and aspects are expected to provide a clear insight into the issues relating to the target strength variability. Such physical models are developed by converting the physiological shape of the fish into standard and simple geometrical shapes. Data obtained from some of the commercially important species, individually positioned at the center of the acoustic beam, 3 m from the transducer in a test facility were used for the computation of target strength. The target strength value obtained from these reference targets is an indicator of the model performance. Mathematical description of the scattering by some of the species and subsequent comparison with laboratory data have demonstrated that the scattered level by an individual due to a single ping, strongly depends upon size, shape, frequency, material properties, and orientation. Perhaps one of the most notable peculiarities of this work is the simplicity of the approximation and the close agreement between the real world value and the model solution.

Development of an underwater target classifier using target specific features

In Sonar, the detection and estimation functions are performed by signal processors, which involve the computation of various statistics, for enhancing the overall performance of the system. This also takes into account all the undesirable propagation effects caused by the underwater channel. Underwater targets can be classified by using certain target specific features such as target strength, target dynamics, and the signatures of the noise generated by the targets. Rough identification of the targets is carried out with target strength values at known aspects while for precise identification, classification clues from target dynamics and target signatures are generated. Databases for the engine noise spectra of various underwater targets, propeller noises, machinery noises and cavitation noises, speed-noise characteristics, etc., have been developed. The signal energy estimated within a finite-time interval is compared with the earlier detection/estimation decisions, which are stored in the target data record and the relevant target data are updated. The algorithm for identification of target from the most matching signature patterns in the database will generate the classification clues, which will help in target identification. Salient highlights of an underwater target classifier using the above-discussed target specific features are presented in this paper.

Target strength and density structure of Hawaiian mesopelagic boundary community patches

A broadband sonar system and digital camera with strobe lights were mounted on a vertically profiling frame with a depth sensor. The target strengths and densities of animals within individual mesopelagic boundary community patches were investigated as a function of depth. Simultaneous echosounder surveys permitted comparison of density estimates from echo‐energy integration and echo‐highlight counting. Target strength values suggest nearshore boundary community animals are primarily myctophid fishes which was confirmed by preliminary photographic evidence. Target strength varied significantly as a function of distance from the shoreline and time. Echo‐energy integration estimates of density made with these revised target strengths compare well with those made with echo highlight counting. These density measures show that previous density estimates were too low but do not change the conclusions of these studies. Vertical microstructure in density was apparent but animal size and compositional structure was not evident within a patch. Patch edges were abrupt, with no differences in the density or target strength from patch interiors. These edges were generally straight, with a sharp drop in density to the background density of zero. Estimates of animal size as a function of time provide information about the diel migration patterns of these mesopelagic animals.

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.

Ex situ target strength of rockfish (Sebastes schlegeli) and red sea bream (Pagrus major) in the Northwest Pacific

Abstract This study determined the ex situ target strength (TS) of rockfish (Sebastes schlegeli) and red sea bream (Pagrus major) in an artificial seawater tank as a means of helping to estimate fishery resources in coastal areas. TS experiments were conducted at frequencies of 38 kHz (split beam), 120 kHz (split beam), and 200 kHz (dual beam). The species were examined under two conditions: first, live fish confined to a small, net cage; and, second, as free-swimming fish inside a large tank. The study examined 21 rockfish and 20 red sea bream. The data were used to obtain expressions for TS against length and weight for the two species. The relationships between TS and fish length were as follows: for rockfish, TS38 kHz = 20 log10(L) − 67.7 (r = 0.80), TS120 kHz = 20 log10(L) − 74.3 (r = 0.61), TS200 kHz = 20 log10(L) − 72.8 (r = 0.41); and for red sea bream, TS38 kHz = 20 log10(L) − 66.8 (r = 0.86), TS120 kHz = 20 log10(L) − 74.0 (r = 0.65), TS200 kHz = 20 log10(L) − 74.1 (r = 0.83). The TS equations for rockfish and red sea bream as a function of fish weight at 38 kHz were TS38 kHz = 6.75 log10(W) − 56.0 (r = 0.78) and TS38 kHz = 4.08 log10(W) − 49.9 (r = 0.89), respectively. For comparison, calculations using the Helmholtz–Kirchhoff ray-approximation model based on swimbladder morphology were compared with the measured TS. When the tilt angle of the fish is zero, the mean TS from the model is 3–10 dB higher than the experimental results, although the maximum TS values were only 3–4 dB different.

A design study of an acoustic system suitable for differentiating between orange roughy and other New Zealand deep-water species

Using the simple slab-cylinder acoustic model for fish, developed by Clay and Horne [J. Acoust. Soc. Am. 96, 1661–1668 (1994)], the target strengths of three of New Zealand’s deep-water fish species, namely orange roughy, black oreos, and smooth oreos, have been derived. The target strengths derived for the model fish have been found to be in reasonable agreement with currently accepted target strength values. These three model fish were used in a study to test the species discrimination of a chirp sonar system. Chirps of center frequencies 40, 80, and 160 kHz and bandwidth of 10, 20, and 40 kHz have been used to acoustically illuminate the three fish species listed above and the matched, filtered responses to the chirps determined. The effect of the response of transducer or system bandwidth has also been investigated. It has been found that the bandwidth of the chirp is much more important for resolving detail in a fish target than the chirp center frequency. A bandwidth of at least 20 kHz, and preferably 40 kHz, produces matched filtered responses for black and smooth oreos and orange roughy which are quite clearly species specific. Results suggest that with orange roughy it may be possible to size and even sex the targets acoustically.

Acoustic observations of jellyfish in the Namibian Benguela

Multi-frequency acoustic data (18, 38 and 120 kHz) were collected in conjunction with pelagic trawl sampling for gelatinous macrozooplankton during a cruise to the Namibian Benguela in September 1999. Sampling focused specifically on the scyphozoan Chrysaora hysoscella and the hydrozoan Aequorea aequorea, both of which occur in large numbers, are probably of major ecolog- ical importance, and physically hamper pelagic fishing and diamond extraction activities. C. hyso- scella was detected predominantly at an inshore station and A. aequorea was found in greatest abun- dance further offshore in deeper water. Echo-sounder observations were linked directly to net catches, and relationships between catch density (number of individuals m -3 ) and nautical area scat- tering coefficients (sA) at each frequency were determined for both species in order to estimate target strength (TS) using the comparison method. TS for C. hysoscella (mean umbrella diameter 26.8 cm) was -51.5 dB at 18 kHz, -46.6 dB at 38 kHz and -50.1 dB at 120 kHz; for A. aequorea (mean central umbrella diameter 7.4 cm) TS was -68.1 dB at 18 kHz, -66.3 dB at 38 kHz and -68.5 dB at 120 kHz. These TS values compared favourably with previously published estimates for related species. Jelly- fish were caught at high numerical densities (maxima 3 C. hysoscella per 100 m 3 , 168 A. aequorea per 100 m 3 ). These high densities, combined with the not unsubstantial TS at frequencies used for fish- eries surveys, imply that jellyfish could potentially bias acoustic estimates of fish abundance. We sug- gest a simple multifrequency approach that could be used to discriminate between echoes from jelly- fish and some commercially important pelagic fish in the northern Benguela ecosystem.

Hydroacoustic differentiation of adult Atlantic salmon and aquatic macrophytes in the River Wye, Wales

Split-beam hydroacoustic techniques have been used to enumerate adult Atlantic salmon ( Salmo salar) passage on the River Wye since 1994. Aggregations of aquatic macrophytes, principally Ranunculus fluitans, are seasonally present at the monitoring site. At high densities, these macrophytes return target strength (TS) values similar to those of adult salmon. Target direction-of-movement information readily resolves upstream-migrant salmon from static- or downstream-traveling macrophytes. However, Atlantic salmon are iteroparous, and downstream-migrant kelts must be factored into the acoustic estimates. Net upstream target movement and/or TS were not reliable indicators of salmon passage at the River Wye site when aquatic macrophyte aggregations were present. In 1995, a subset of data ( n = 71) was selected from the ongoing monitoring program to determine the feasibility of resolving salmon from macrophytes using acoustic parameters other than target direction-of-movement or mean TS. Individual targets were visually identified as either salmon or macrophytes based on concurrent video records. Distributions of available acoustic parameters were statistically compared between the two target-types using a two-tailed t-test assuming equal variance ( P = 0.05). Available acoustic parameters included echo position, pulse width (at –6, –12, and –18 dB power points), amplitude, beam pattern factor and target strength. Pulse width standard deviation (PWSD) measurements were determined to be the most effective individual parameters for discriminating macrophytes from salmon. Based on PWSD at the –6 dB echo power points, 77% of all macrophyte targets were removed from the mixed data set. Multiple selection criteria increased total target discrimination. Applying a combination of –6 and –12 dB PWSD, and Y–axis target slope criteria rejected 94% of all macrophytes, retaining all salmon targets.

Typology and behaviour of tuna aggregations around fish aggregating devices from acoustic surveys in French Polynesia

Eighty-seven two-hour acoustic surveys (radius 0.8 nautical mile, vertical range 0–500 m) around 17 fish aggregating devices (FADs) were conducted in French Polynesia between December 1995 and February 1997. Associated tuna densities were calculated using two different techniques: echo counting when the fish had sufficient distances from each other and echo integration when the fish swam close together (in schools). No acoustic detection of tuna was observed during 27 of the 87 surveys, representing 81 % of all the nocturnal surveys and 15 % of the diurnal ones. The 60 other surveys showed three different classes of aggregations: (1) ‘deep scattered fish’, observed 45 times, (2) ‘intermediate scattered fish’, observed 16 times, and (3) ‘shallow schooling fish’, observed 16 times. Sometimes aggregations of different classes were observed beneath the same FAD. The size of the fish inside the aggregations (determined from target strength values), the distance between the individuals, and the depth of the fish all decreased from ‘deep scattered fish’ to ‘shallow schooling fish’ (100–300 m for ‘deep scattered fish’, 50–150 m for ‘intermediate scattered fish’, and above the depth of 50 m for ‘shallow schooling fish’). Fish densities also varied according to the class of aggregations: 7.3, 26, and 801 fish per km 3 on average for ‘deep scattered fish’, ‘intermediate scattered fish’, and ‘shallow schooling fish’, respectively. The highest densities were observed during daytime, while night-time observations indicated a variety of situations, from the absence of individuals to large amounts of fish.

Problems with acoustic target strength measurements of a deepwater fish, orange roughy (Hoplostethus atlanticus, Collett)

In situ target strength measurements of fish at 600 to 1200 m depth were made around a spawning aggregation of orange roughy oV 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 myctophidfishes with standard length modes at 8.2 and 5.3 cm; thesefish 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. ? 1997 International Council for the Exploration of the Sea

Depth dependence of target strength of live kokanee salmon in accordance with Boyle's law

The target strength (TS) offish varies with a number of factors, especially the presence or absence of a swimbladder. This study investigated the eVect of the swimbladder on the TS of live kokanee (Oncorhynchus nerka) in relation to depth. Experiments were conducted at Lake Kuttara, Hokkaido, Japan, which is abundant with kokanee. Fish samples were caught with a fishing net. The live fish were suspended on the sound beam axis, and tilted from 50) (head-down aspect) to +50) (head-up aspect). The dorsal aspect TS functions were measured at 50 kHz. Measurements were performed when the fish were forced to change depth rapidly. The TS values were measured as a function of tilt angle and depth, and the maximum dorsal aspect TS (TSmax) values and the averaged dorsal aspect TS (TSavg) values were calculated and compared. TSavg values were calculated with respect tofish tilt-angle distribution, which was assumed to have a mean value of 5) and a standard deviation of 15). Both TSmax and TSavg decreased with depth in accordance with Boyle’s law, i.e. a reduction rate of 6.7 dB per 10 atm was observed. ? 1996 International Council for the Exploration of the Sea