Calibration of acoustic instruments for fish density estimation: a practical guide

The acoustic estimation of fish biomass, as currently practiced, requires calibrated equipment. A good calibration is essential to good biomass estimates although it cannot guarantee these. We describe how to perform a highprecision calibration by means of a standard target sphere. This involves primary measurements of three quantities: (1) on-axis sensitivity of the overall echo-sounding and integrating system, (2) time-varied-gain function of the receiver, and (3) equivalent beam angle of the transducer. If the transmitter, transducer, receiver or echo-integrator perform poorly, however, a good calibration is not possible. Measurement of the electrical properties of the system components is therefore included as an element of calibration. In this paper each of the four mentioned elements of calibration are described both in principle and in practice, with detailed examples drawn from experience. Calibration accuracy and inter-ship calibration are also described in some detail. In conclusion, the provisional nature of this work is emphasized, as new developments will undoubtedly continue to improve on present techniques.