Title Application of dolphin’s sonar abilities to echo sounders
Authers Tomohito IMAIZUMI
Keywords Sonar of dolphin, broadband sonar signal, target strength of fish, target strength spectrum
Citation Bull. Fish. Res. Agen. No.28, 47-113, 2009
Conservation and management are necessary for sustainable use of fisheries resources. For this purpose, acoustic surveys of fisheries resources using scientific echo sounders are conducted worldwide. Although these surveys are powerful methods of estimating fish stock biomass directly over wide areas, the acoustic survey method has several drawbacks. One is the lack of a definitive method for classifying fish species using echo data and another is that in situ target strength (TS) measurement methods can only be applied to resolvable fish at short range or in sparse fish schools. In contrast, dolphins detect, pursue, and prey on fish using their excellent sonar capability known as “echolocation”. Dolphins can recognize not only the target size, but also its material and shape. As fish are the main target of dolphins, this sonar capability is applicable to the improvement of artificial sounders used in acoustic surveys and selective fishing.
 The present study was performed to investigate the application of a dolphin’s sonar abilities to echo sounders. In addition, this study focused on the characteristics of broadband frequency and short duration of dolphin sonar sound and measurement of target strength (TS) spectra of fish using this type of sonar. The broadband frequency characteristics of the TS spectra of fish are believed to be key factors in target discrimination and behavior observation.
 First, a system for measuring TS spectra was developed and the accuracy of the TS spectra thus obtained was confirmed using a spectral ratio method based on the ratio of the amplitude spectrum of the reflected and incident waves. The TS spectra of metal spheres could be calculated accurately using a theoretical model, and their backscattering was unidirectional. The frequency characteristics of the scattering of metal spheres agreed closely with the theoretical calculations in a water tank. Therefore, the system and spectrum ratio method are capable of measuring TS spectra accurately.
 Second, the TS spectra of anesthetized fish from three species, horse mackerel (Trachurus japonicus), sea bream (Pagrus major), and chub mackerel (Scomber japonicus), were measured in a water tank. The variation in TS spectra with respect to the tilt angle of the fish was also measured for one sea bream. The TS spectra of fish varied considerably among species, individuals, and tilt angles.
 Third, the TS spectra of tethered live fish were measured at sea. As we could not confirm the relationship of the position between the transducers and the fish in the sea experiment and because measurements are affected by the directivity functions of transducers, measuring the TS spectra accurately was difficult.
 To compensate for the directivity, the fish were suspended with two metal spheres, and directivity was compensated using echoes from the metal spheres. The variations in the TS spectra of the tethered swimming fish (chicken grunt, Parapristipoma trilineatum) were observed continuously. The TS spectra of fish with tilt angle changes are considered useful for species discrimination.
 Furthermore, because of the very short pulse of the dolphin-like sonar, we could obtain well separated single echoes that would otherwise be multiple echoes for ordinary echo sounders. The sea experiments suffered from directivity problems. We attempted to develop a broadband split-beam (BSB) system to compensate for the directivities of transducers. The split-beam (SB) method can measure the position and direction of the target from the transducer using the time differences calculated from the echoes. These echoes are received by the quadrant arrays of the receiving transducer. The SB method using the broadband signal was first applied. The time difference detection methods were simulated, and we decided to use the cross-correlation method. Second, we confirmed that the measured directivity functions at four sample frequencies agreed with the theoretical directivity function calculated from the positions of the array elements. Third, to confirm the utility of the BSB system, we measured the frequency characteristics of metal spheres at sea. By applying a directivity correction to the measurements, good agreement was obtained between theory and measurement. Finally, we measured consecutive in situ TS spectra of fish at sea using the BSB system with high range resolution.
 Next, to determine the advantages of the dolphin sonar signals, we examined the sonar signals of dolphins and artificial tone burst waves of the same duration as dolphin-like sonar signals. In the experiment performed at sea, echoes from metal spheres measured using different signals (i.e., the dolphin-like sonar signals and short tone burst waves) were crosscorrelated with each incident waveform as a reference or replica.
 The signal-to-noise ratios (SNRs)were then compared with those of the original data. The SNRs improved by more than 10 dB for all sonar signals. The SNRs were compared after cross-correlation, and the results indicated that the dolphin-like sonar signals improved the SNR by 9 dB (at maximum) over the tone burst wave. Although broadband signals are susceptible to noise, the dolphin sonar signals have greater potential than the other types.
 The findings of the present study will contribute to the development of a system and method of accurately measuring broadband TS spectra of targets with high range resolution. The variations in in situ fish TS spectra originating from movement and changes in tilt angle of the target may represent information that will allow species discrimination as well as estimation of size and behavior. We can measure accurate TS spectra of the target using dolphin-like sonar signals with short duration and broadband frequency characteristics. This will help resolve the problems associated with acoustic surveys and contribute to the management of fisheries resources.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull28/imaizumi.pdf