The multi-frequency acoustic method to identify fish species using the frequency dependence of backscatter from fish (relative frequency response) has been investigated. The method has been successfully applied to broad identification, such as distinguishing between swimbladder and nonswimbladder fishes, but it is not always possible to identify acoustically similar fish species as swimbladder fishes. To improve identification power, we, therefore, propose a method that uses the difference of fish-length-to-wavelength ratio (L/λ) characteristics of backscatter among fish species (relative L/λ characteristic), instead of or together with the relative frequency response. We, first, theoretically confirm the rationale for using the relative L/λ characteristic with prolate-spheroid scattering models of fish. Second, we compute target strengths for several types of fish models, and show the advantage of using the relative L/λ characteristic. Third, using experimental examples in which species identification was difficult, we apply the proposed method and ascertain its effectiveness. Finally, we discuss the necessary and challenging procedure to know fish lengths and other issues.
Last year, a high-data-rate digital underwater acoustic mobile communication experiment was conducted in the dep sea water (DSW) of the north and south area of Suruga Bay, Japan. The acoustic communication signal, which utilized a single-carrier modulation, was transmitted from an acoustic source (15–25 kHz) on a manned vehicle moving in DSW to a surface-vessel. Initial data analysis demonstrates that a maximum data rate of 70 kbps could be achieved over a 3.6 km range using 128 quadrature amplitude modulation (QAM), which far exceeds the performance of past published experimental results and commercial acoustic modems.