Abstract
General trends of the target strength of fish or other organisms are investigated by using prolate spheroidal scattering models. The soft spheroid is taken as the model of the swim bladder of fish, and the liquid spheroid for the fish body without bladder. Backscattering amplitudes are calculated for the spheroids with typical physical parameters of actual fish. They are shown as the frequency and size dependences, the backscattering patterns, and the density and sound speed dependences. These results are compared with experimentally observed target strengths of sardine, mackerel, krill, etc. A good agreement was found. The dorsal-aspect target strength of bladder fish shows little frequency dependence and is approximately proportional to the squared body length. Almost all the backscattered energy is attributed to the bladder. The target strength of the fish or organisms without a bladder is very small. Their swimming orientation and physical parameters of the body considerably affect the scattering to cause variability in the target strength.
