The target strengths of fish

Abstract Knowledge of the acoustic target strength of a fish (or of a shoal of fish) is required to enable the performance of present and future sonar equipments to be determined for fish targets. Also, it is hoped that measurement of the strengths and characters of the echoes received from fish will give a reliable guide to the size of the fish and, possibly later, an indication of at least gross differences between various types of fish. As a result, a variety of workers have been interested in the acoustic scattering properties of fish, initially at the frequencies of existing equipments, but later over a much wider range of frequencies, pursuing a more fundamental approach to understanding the formation of fish echoes and their relation to the physical structure of the fish. Over the last 12 years, a programme of research in this field has been directed by the author. Recently, the White Fish Authority has supported the design of apparatus operating at 30, 97, 170, 278 and 547 kHz and measurements have been made on cod fish at these frequencies in a large water tank. This field is also related to a number of other subjects such as the design of new sonar equipments, the identification, counting and behaviour of fish, and telemetry from the trawl. Over 50 references are given. Further work remains to be done.