Masahiko Furusawa

Measuring the target strength spectra of fish using dolphin-like short broadband sonar signals

Dolphins identify their prey using broadband sonar signals. The broadband spectrum of the target strength (TS) of fish is believed to be a key factor in target discrimination. In this study, the TS spectrum was measured using sonar signals generated by two different dolphin species: finless porpoise and bottlenose dolphin. First, the broadband form functions of a tungsten carbide sphere and a copper sphere were measured in a water tank, and a close agreement between measurements and theoretical values was confirmed. Second, the TS spectra of anesthetized fish from three species were measured in a water tank. Although the results showed characteristics similar to previous measurements, they varied among species, individuals, and tilt angles. Third, the TS spectra of live fish suspended and tethered by nylon monofilament lines were measured at sea. The dolphin-like sonar signals were effective in obtaining the broadband TS spectra of the fish. Cross-correlation processing of the echo from a tungsten carbide sphere showed a further advantage of using the dolphin-like sonar signals: the signal-to-noise ratio increased by more than 10 dB. The variation of TS spectra with fish behavior provides useful information for target identification.

Attenuation of sound by schooling fish

Attenuation of sound by schooling fish was measured or estimated for yellowtail (Seriola quinqueradiata), sea bream (Pagrus major), and spotted mackerel (Scomber australasicus) in culture nets and a net cage. The results, along with those of other researchers, are normalized to obtain extinction cross sections per 2/3 power of the body weight. In the normalized cross sections there are no large differences among fish species, sizes, and measuring methods. The attenuation increases slightly with the frequency. Attenuations to be encountered in field observations by echo sounders are estimated and the results reveal that ordinary fish schools do not attenuate sound significantly.

Exact Simulation of Fish School Echoes and its Applications

A numerical simulation method was developed to investigate fish school echoes. The simulation incorporates strict expressions for fish target strength model, directivity of a transducer, and echo waveform in order to realize high fidelity. The fish school echoes are constructed by synthesizing single echoes for each ping. Two application examples are shown. One is the confirmation of linearity between the distribution density offish and the simulated volume backscattering strength; the result showed a very good linear relationship. The other example is fish school feature extraction by the wavelet analysis. In the results, no apparent one-to-one correspondence between the given distribution pattern and the results of the wavelet analysis ,but a characteristic wavelet pattern is observed.

Quantifying Sea Bottom Surface Backscattering Strength and Identifying Bottom Fish Habitat by Quantitative Echo Sounder

A quantitative echo sounder (QES) was used to measure the bottom surface backscattering strength (SS) extensively off Southern Jawa Island, Indonesia. The average SS by the bottom echo integration method were plotted as a map with the depth contour to give a synoptic view of the topography. On the other hand, the results from the ring surface scattering (RSS) model gave us more detailed SS values in ping base. A simultaneous display of the fish volume backscattering strength and the bottom SS was useful to observe bottom fish habitats remotely. The bottom material estimated by the measured SS showed that the fish schools were abundant in the sand bottom in this area.

Experimental Investigation of an Acoustic Method to Estimate Fish Abundance Using Culture Nets

Using an acoustic method fish abundance in many culture nets has been measured and the results statistically analyzed. The acoustic system consists of two parts, an echo-integrator which gives distribution density and a specially designed sectorscanning sonar which provides distribution volume. By matching the measured numbers to actual ones, parameters concerning the target strength of fish and the attenuation by schooling fish are derived. These parameters are compared with actual observations.

Analysis of the temporal structure of fish echoes using the dolphin broadband sonar signal

Behavioral experiments indicate that dolphins detect and discriminate prey targets through echolocating broadband sonar signals. The fish echo contains components from multiple reflections, including those from the swim bladder and other organs, and can be used for the identification of fish species and the estimation of fish abundance. In this paper, temporal structures were extracted from fish echoes using the cross-correlation function and the lowpass filter. First, the echo was measured from an anesthetized fish in a water tank. The number, reflector intensity, and echo duration were shown to be dependent on the species, individual, and orientation of the fish. In particular, the echo duration provided useful information on the fish body height and for species identification. Second, the echo was measured from the live fish suspended by nylon monofilament lines in the open sea. It was shown that this duration could be estimated regardless of whether or not the fish were moving.

Measurement of sea bottom surface backscattering strength by quantitative echo sounder

Measurement theory and method of the bottom surface backscattering strength (SS) using a quantitative echo sounder (QES) are discussed and applied in the ocean near Java, Indonesia. The frequencies of the QES were 38, 70, and 120 kHz. The measurements of bottom echoes and sampling of bottom material by a dredge were done simultaneously. Bottom characterization was based on analysis of particle size distribution for bottom samples taken during the survey. The SS value increases with the increase of the mean diameter of the bottom particles. The SS decreased with increasing frequency. The effectiveness of QES for measurement of the SS along with observation of the depth topography were demonstrated.

Validation of multi-frequency inversion method by using dummy scatterers of zooplankton

High- and multi-frequency acoustic measurement systems and the multi-frequency inversion (MFI) method have been used to measure spatial distributions and abundances of zooplankton by size. In this study, the calibration method for high- and multi-frequency systems was developed and the validation of MFI method was carried out by scatterer measurement. The standard sphere calibration method that has not been applied to such high- and multi-frequencies was applied to calibrate our high- and multi-frequency system, TAPS-6 (Tracor Acoustic Profiling System, BAE Systems). An optimum size of standard sphere of tungsten carbide of 1 mm radius was derived to have a small target strength variation for the six frequencies of TAPS-6, and the practicability and precision of the standard sphere calibration method was confirmed for those frequencies. A school or cluster of dummy scatterers of zooplankton with small tungsten carbide spheres were designed to validate the MFI method, and volume back-scattering strength values were measured by the multifrequency system. By comparing the result of the inversion with their real composition, the features of the MFI method could be validated and examined.

Limitations in measurements of acoustic scattering from fish at rather high frequencies

Recent years, the frequencies used in quantitative echo sounders have become multiplicative for the purposes of fish species identification, simultaneous observation of fish and plankton, etc. For example, frequencies of 18, 38, 70, 120, 200, and 364 kHz are used. At frequencies above 100 kHz, however, the absorption attenuation increases, and therefore the signal-to-noise ratio (SNR) becomes small and the complete correction of the attenuation is difficult to result in sometimes a large measurement error. This paper elucidates the SNR and the error and clarifies the limitations and cautions of the high frequency measurements. From the viewpoint of the SNR, a measurement of fish at 200 m depth is possible at up to 200 kHz, but a correction of the absorption becomes abruptly difficult above 70 kHz. Therefore fish scattering measurements at high frequencies need the absorption coefficients matched to environment.

Robust analysis of single echo trace obtained by split beam sounder

The theory and method of the echo trace analysis of single echoes (ETA), which simultaneously analyze the shape and level of single echoes, are almost established. But, the method has not been extensively used. The major reasons are high‐quality echoes with a high signal‐to‐noise ratio (SNR) necessary for the analysis are some times difficult to obtain; the error introduced by transducer motion is large; and conditions for the theory sometimes could not be fulfilled. Therefore, we apply the ETA to the split beam method, which is most advantageous for the ETA, and make the method robust by introducing the following: careful echo selection considering SNR, correction of the effect of transducer motion, smoothing of the trace by regression analyses, introduction of indexes checking reliability, etc. The useful results of our ETA are the track, the swimming speed and orientation, the target strength as a function of tilt angle, and the estimated size of individual fish. The robust ETA was applied to echoes of...