A comparative analysis of in situ methods for directly measuring the acoustic target strength of individual fish

In recent years, acoustic techniques have been used extensively to monitor and quantify fish stocks remotely. One of the parameters that is often measured is the target strength of the individual fish in the population. To obtain the target strength of a fish from its acoustic echo, the effect of the transducer beam-pattern factor must be removed from the echo amplitude. This paper discusses a number of different approaches for directly estimating the beam-pattern factor for an individual fish target. The effects of noise and interfering signals on the various estimation techniques are investigated. Expressions are derived for the bias and variance in the estimates of the beam-pattern factor as a function of signal-to-noise ratio and angular location of the target. Results of a Monte Carlo simulation of the effect of interference on the various processing techniques are presented. Some of the problems encountered in implementing the various techniques are discussed.