The essential aim of biological sampling is to obtainsamples that are representative of the targets underconsideration. Sampling in support of in situ target strength studies should be aimed closely at those targets from which acoustic information is being collected. Other components of the population and contamination of the sample should be avoided if at all possible. Thus if the acoustic measurements are being made on spawning concentrations, it is pointless for the biological sampling to include immaturefish. Similarly, when sampling in a two-layer situation,where the TS data were collected only from the lowerlayer, it is quite possible that incidental catches from theupper layer may contaminate the trawl sample. The idealstudy location would only contain a single species with anarrow unimodal size distribution. The overall programmeof target strength studies and the associated samplingshould, however, be aimed at providing information on alllife history stages of the species relevant to the populationbeing surveyed.It is important to recognise that TS estimation requiresprecise sampling only of the stock component thatcontributes to the TS measurements, whereas acousticabundance surveys require sampling which is aimed atproviding information on the entire target population.