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The utilization of broadband quantization data increases the possibility of practical applications for fish target recognition based on the acoustic scattering theory. However, the quantification of broadband data is more complex than that of narrowband systems, requiring consideration of the broadband characteristics of split-beam transducers, seawater absorption, and circuits. This paper elucidates the scatterer acoustic field equation, transducer power equation, and signal processing flow in split-beam broadband quantization technology for engineering applications. A broadband calculation model based on transducer parameters is proposed to enhance the generalization ability of broadband quantization technology to different types of sonar. The classical echo integration method is combined with Fourier transform to meet the requirements of target strength (TS) estimation under broadband signals. This paper includes a series of experiments to prove the rationality and effectiveness of the method. The results demonstrate that the provided calculation model can more accurately reflect the backscattering characteristics of the scatterer, but certain errors remain. This article analyzes the sources of errors and validates the effectiveness of the new TS calculation method. [ABSTRACT FROM AUTHOR]

Obtaining absolute seafloor backscatter measurements from hydrographic multibeam echosounders is yet to be achieved. We propose a low-cost experiment to calibrate the various acquisition modes of a 30-kHz Kongsberg EM 302 multibeam echosounder in a range of water depths. We use a 38-kHz Simrad EK60 calibrated fisheries split-beam echosounder mounted at 45° angle on the vessel’s hull as a reference for the calibration. The processing to extract seafloor backscatter from the EK60 requires bottom detection, ray tracing and motion compensation to obtain acceptable geo-referenced backscatter measurements from this non-hydrographic system. Our experiment was run in Cook Strait, New Zealand, on well-known seafloor patches in shallow, mid, and deep-water depths. Despite acquisition issues due to weather, our results demonstrate the strong potential of such an approach to obtain system’s absolute calibration which is required for quantitative use of backscatter strength data. [ABSTRACT FROM AUTHOR]

Management of shellfish resources requires a spatial approach where mapping is a key tool. Acoustic techniques have been rarely used to map infaunal organisms with a patchy distribution. We propose and test the use of split-beam echosounder angular information to assess razor shell presence and relative density. Our statistical approach combines textural analysis of angular echograms, standard unsupervised multivariate methods and hierarchical classification through dendrograms to identify groups of locations with similar clam densities. The statistical analyses show that the classification is consistent with groundtruthing data and that results are insensitive to boat motion or seabed granulometry. The method developed here constitutes a promising tool for assessing the relative density of razor clam grounds.

The increased use of side-looking echosounder surveys in large rivers necessitates the creation and evaluation of standardized sampling protocols to effectively sample these habitats. Using data from five Ohio River pools, we evaluated the effects of habitat and transect direction on the quality and quantity of mobile, side-looking, split-beam echosounder data to determine how these factors may affect survey design in large rivers. Split beam echosounder data were collected using two multiplexing 200 kHz transducers offset in angle to maximize water column coverage. Using mixed-effects models, we evaluated the effects of habitat (main channel vs. tributary) and transect direction (upstream vs. downstream movement of the vessel) on fish density and the number of targets per fish track (series of acoustic targets from an individual fish). We also used a skewness test in conjunction with a Kolmogorov-Smirnov test to determine the distribution of fish orientations relative to the transducer and examine differences in the distribution of fish orientations among habitats, transect directions, and years. Mean fish densities were greater in tributary habitat relative to main channel habitat, whereas main channel habitat had a higher mean number of targets per fish track when compared to tributaries. Upstream-facing transects had greater mean fish density estimates and mean number of targets per fish track than downstream-facing transects. Although most fish were oriented perpendicular to the transducer, we identified habitat, transect direction, and annual differences in the distribution of fish orientations. These results suggest that researchers and managers that plan to use side-looking echosounders to assess fish populations in large rivers should ensure that standardized protocols include main channel and tributaries, and are expanded to encompass other relevant habitats (e.g., backwaters and side channels). Further, managers should use fish orientation data to select or develop TS-L equations that best matches the fish orientation distribution of each survey to ensure accurate density and size estimates. [ABSTRACT FROM AUTHOR]

Weber, T. C., Peña, H., and Jech, J. M. 2009. Consecutive acoustic observations of an Atlantic herring school in the Northwest Atlantic. – ICES Journal of Marine Science, 66: 1270–1277.Several successive images of the same school of Atlantic herring (Clupea harengus) were collected over the course of ∼1 h just north of Georges Bank in the Northwest Atlantic. Although the fish may not have been in their natural, undisturbed state, we observed what appeared to be the fish school fragmenting and dispersing, using a split-beam and a multibeam echosounder. Calibrated, 38 kHz, split-beam echosounder (Simrad EK60) and trawl-catch data provided accurate measures of the fish density beneath the vessel. Uncalibrated, 400 kHz, multibeam-echosounder (Reson 7125) data provided synoptic observations of the fish school including estimates of the school volume, morphology, and behaviour. Observations of the angular dependence in the multibeam-echosounder measurements of backscatter from fish allow investigation of the efficacy of extrapolating fish-school densities measured by the split-beam echosounder to the entire school. [ABSTRACT FROM PUBLISHER]

Handegard, N. O., Pedersen, G., and Brix, O. 2009. Estimating tail-beat frequency using split-beam echosounders. – ICES Journal of Marine Science, 66: 1252–1258.Data from a standard split-beam echosounder are used to estimate tail-beat frequency within a dense herring (Clupea harengus L.) layer. The data were collected by lowering a horizontally projecting 38 kHz split-beam transducer into a herring layer at 245-m depth. Individual targets were concatenated into tracks, and a fast Fourier transform was used to estimate the periodogram of the backscattering strengths along each track. A simple model, assumed accurate under certain conditions, was used to relate the periodogram to tail-beat frequency. The requisite conditions are discussed. Examples of accurate tail-beat estimates from single high-quality tracks, which are consistent with statistics on all tracks in the test dataset, are presented. In addition, the cross periodogram between closely spaced individuals was calculated on selected tracks to estimate the relative phase between the adjacent tracks within the school. Finally, the potential use of this method to resolve schooling behaviour, improve target-strength estimates, and even estimate fish condition, is discussed. [ABSTRACT FROM PUBLISHER]

Gurshin, C. W. D., Jech, J. M., Howell, W. H., Weber, T. C., and Mayer, L. A. 2009. Measurements of acoustic backscatter and density of captive Atlantic cod with synchronized 300-kHz multibeam and 120-kHz split-beam echosounders. – ICES Journal of Marine Science, 66: 1303–1309.Effective management strategies for Atlantic cod (Gadus morhua) in the Gulf of Maine require stock assessments based on accurate estimates of its abundance and distribution. If multibeam echosounders are to provide data for such estimates, the relationship between acoustic backscatter and fish biology must be better understood. Working towards this goal, a series of acoustic measurements was made using a 120 kHz, split-beam echosounder (Simrad EK60) and a 300 kHz, multibeam echosounder (Kongsberg EM3002). The transducers from both systems were fixed to a platform over a submerged 98 m3 cage made of 5 cm stretched-nylon mesh. After standard-sphere calibrations, the cage was stocked with live, mature Atlantic cod, with a mean total length of 80.7 cm (range: 51.5–105.0 cm). The echosounders synchronously collected acoustic data, while the cod were monitored with two underwater video cameras. Cod were incrementally removed from the cage to provide a time-series of acoustic backscatter at four densities (n = 128, 116, 66, and 23). Backscatter measurements of cod are compared between echosounders and over time, and the factors affecting the acoustically derived density estimates are discussed. The benefits and limitations of the EM3002 are highlighted. [ABSTRACT FROM PUBLISHER]

Peña, H. 2008. In situ target-strength measurements of Chilean jack mackerel (Trachurus symmetricus murphyi) collected with a scientific echosounder installed on a fishing vessel. – ICES Journal of Marine Science 65: 594–604.The use of commercial fishing vessels for scientific purposes has increased worldwide in the past 10 years. Many such studies have involved the collection of acoustic data from both uncalibrated and calibrated echosounders. However, few studies have involved investigations of in situ target strength (TS). During August/September 2003, in situ TS data on Chilean jack mackerel (Trachurus symmetricus murphyi) were collected on board a commercial purse-seiner during normal fishing operations off Chile, using a 38 kHz, Simrad EK60 scientific echosounder. The single-target detections of Chilean jack mackerel were filtered by depth, off-axis beam angle, and beam-compensation criteria to improve the quality of the data used for the TS calculations. Two methods, using raw data and tracked data, were employed to calculate the mean acoustic-backscattering cross section (σbs) and mean TS of Chilean jack mackerel. The results of the two approaches gave similar results, with a strong mode in the mean TS distribution between −35 and −37 dB for fish lengths ranging from 26 to 34 cm, indicating a b20 value of −66 dB. These results agreed well with most results published for this species and others of the same genus (i.e. T. trachurus, Trachurus t. capensis, and T. japonicus). [ABSTRACT FROM PUBLISHER]

Burwen, D. L., Nealson, P. A., Fleischman, S. J., Mulligan, T. J., and Horne, J. K. 2007. The complexity of narrowband echo envelopes as a function of fish side-aspect angle. – ICES Journal of Marine Science, 64: 1066–1074. High-frequency, narrowband acoustic signals may contain more information on fish size and orientation than previously thought. Our observations of dual frequency identification sonar (DIDSON) images of fish orientation paired with split-beam echo envelopes helped clarify why metrics such as echo duration have performed better than target strength measurements when predicting salmon lengths at side aspect. Fish orientation has a pronounced effect on the duration and shape of split-beam echo envelopes from large (80–130 cm) salmon insonified at side aspect. At near-normal aspect angles, echo envelopes are unimodal, symmetrical, and resemble echo envelopes from calibration spheres. With increasing oblique-aspect angle, echo shapes become less symmetrical as the number of peaks increases, and echo duration and amplitude become more variable. Using angle and range coordinates, peaks in an echo envelope can be traced to their origin on a DIDSON image. At oblique-aspect angles, discrete peaks develop that are reflected from regions close to the head and tail. In addition, the distance between peaks increases with increasing aspect angle and is larger than can be explained by swimbladder length. [ABSTRACT FROM AUTHOR]

Abstract: A robust, remotely controlled transducer platform has been designed for use in stationary shallow water acoustic surveys. The transducer array platform is integrated with a weatherized top-side box allowing for collection of acoustic data at remote stations. The platform allows for vertical placement of the transducer array in the water column and for remotely manipulating the transducer pitch. Both a dual-frequency identification sonar (DIDSON) and a BioSonics split-beam transducer were mounted to the platform for simultaneous data collection over a variety of estuarine habitats. The platform is quickly deployed and retrieved by a single user from a small vessel. Effective water depths for optimal use range from 1 to 3.5m; however, the equipment can be easily deployed at greater depths. The array platform has been in service for 2 years and the performance level and data quality remain high. [Copyright &y& Elsevier]

During acoustic surveys for biomass estimation, fish aggregations near the seabed may not be correctly measured due to false detection of the bottom echo. The extent of this problem depends on the spatial and density features of the near-seabed aggregations. We conducted experiments using a tower structure deployed close to the bottom of a sea loch. The tower is 10 m high with a split-beam transducer at the top and a fish cage at the bottom. The effect of a bottom slope is simulated by tilting the transducer. In experiments with various densities and sizes of gadoids in the cage, echoes from the vicinity of the seabed were studied over hard and soft ground. In addition to the range, the split-beam echosounder gives two angular coordinates of the target direction. These may be combined into one measure of angular displacement, namely, the angle between the apparent target direction and the transducer axis. We call this the split-beam angle (SBA). We found that the SBA is not necessarily an accurate indication of the target direction. Echoes from fish aggregations and the seabed have different characteristics in this respect. When the seabed echo is detected with few interfering targets above, the SBA is an accurate indication of the seabed slope, and assuming the slope does not change over a short series of pings, the SBA is highly correlated. On the other hand, the SBA from multiple fish echoes is highly variable, as expected, and the ping-to-ping variation is essentially random. Furthermore, when the seabed echo is transmitted through a substantial density of fish, the interference can change the SBA, although the ping-to-ping correlation of the seabed SBA remains superior to that of fish aggregations. We also studied records from acoustic surveys on various research vessels to provide comparable results at full scale. When there is a low density of near-seabed fish, the correlation between the fore-aft SBA and the seabed gradient is optimal at the start of the first seabed echo; it declines at sub-bottom ranges. When there are dense aggregations of fish near the seabed, the automatic bottom-detection algorithm may be located on top of the aggregation, so that the echo integration misses a substantial quantity of fish. Examples from acoustic surveys in the North Sea are presented to illustrate this problem. [Copyright &y& Elsevier]