INTERPRETATION OF IN-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 distrib ution from trawl catches. The acoustic and fishing data are directly c omparable only if they are samples from the same population, but the T S measurements depend on the detection of isolated targets which may n ot be representative of the ensonified population. Errors may occur th rough the false detection of multiple targets, and the size-dependence of schooling density means that larger fish are more likely to be det ected. We show that such effects can result in the estimated TS being biased high. We describe a stochastic method for estimating the consta nt b in the formula TS=20 Log (L)-b, by a least-squares fitting proced ure. 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 Diplotaxodon spp. around 15 cm in length, b is 67.4 +/- 0.2 dB. (C) 1996 International Council for the Exploration of the Sea