Interpretation ofin situ target-strength data

Target strength (TS) may be estimated as a function of fish length (L) by comparing in situ acoustic measurements with the fish size distribution from trawl catches. The acoustic and fishing data are directly comparable only if they are samples from the same population, but the TS measurements depend on the detection of isolated targets which may not be representative of the ensonified population. Errors may occur through the false detection of multiple targets, and the size-dependence of schooling density means that larger fish are more likely to be detected. We show that such effects can result in the estimated TS being biased high. We describe a stochastic method for estimating the constant b in the formula TS=20 Log (L)−b, by a least-squares fitting procedure. It is assumed that the echo-amplitude has a Rayleigh distribution. The method has been applied to data from surveys of cichlids in Lake Malawi. From 33 independent measurements of Diplotaxodonspp. around 15 cm in length, b is 67.4±0.2 dB.