David E. Weston
White Laird, 77 Wyke Rd., Weymouth, Dorset DT4 9QN, England
Early work on long-range sonar displays of fish is reviewed, for observations in the Perranporth sea area. There is a concentration on the complications in interpretation arising from the use of a variety of pulse types and carrier frequencies. A useful technique was the interleaved transmission of different pulses or frequencies, permitting a comparison of virtually synoptic records. For linear frequency-modulation pulses there are dramatic changes in display appearance when pulse duration or pulse bandwidth are changed, which must be allowed for in the inversion giving fish distribution and behavior. There are further differences and problems with noise-modulation pulses; although these can perform well, they do often give very poor pictures. In contrast to the pulse-type effects there is a relatively small dependence of detailed appearance on carrier frequency for frequencies 1, 2, and 3 kHz; reflecting the slow dependence of fish school target strength on frequency. Of course system performance does depend on frequency as it does on the whole system engineering. In spite of the complications it is possible to find useful information on fish numbers, school numbers, movement, target depth, and behavior generally.