Direct laboratory measurement of forward scattering by individual fish

Acoustic backscatter has been widely used to detect and quantify fish. However, other types of information can also be exploited for this purpose. One is acoustic signals scattered by fish in the forward direction. This paper describes a novel laboratory experiment aimed at direct measurement of forward scattering by individual fish. A copper sphere is first used to test the measurement concept and establish experimental procedures. Then forward scattering by fish, in both the dorsal aspect and side aspect, is measured against the incident angle of sound, and simultaneous measurements of backscatter are also made. The results indicate that the forward scattering of individual fish is significantly stronger than the backscattering: At or near normal incidence, the forward scattering is 14–17 dB stronger. It is also found that the forward scattering varies with the incident angle of sound to a much less extent than the backscattering. These features could prove to be the advantages of using forward scatter for detection and quantification of fish. How to make the best use of forward-scattered signals remains to be exploited.