The echo-sounder used in the exercise was a Simrad EY-M. This echo-sounder has been specially designed for freshwater fish stock assessment, has two time varied gain controls, and the signal can be recorded on tape for later analysis. Simultaneously as the signals are recorded on the tape, they are reproduced on a six-inch straight line paper echo-gram recorder. The working frequency is 70 kHz and the duration of the transmitted pulse is 0.6 ms. This gives a depth resolution of about 0.8 m. A discussion of acoustic methods in fish stock assessment is given by Mathisen (1980).
The transducer was mounted on a V-fin (Figure 4) to give stability when being towed from a small boat, and was towed at a depth of between 1.5 m and 2 m, which kept it away from surface bubbles and the noise of the motor and propeller. The transmitted beam angle was 11° measured at the -3 dB level, and the side lobes were 15–20 dB down compared with the main lobe.
The echo data were recorded on good quality C-90 magnetic tape cassettes on a Nakamichii 550 tape recorder.
