Biological sampling

The essential aim of biological sampling is to obtain samples that are representative of the targets under consideration. 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 immature fish. Similarly, when sampling in a two-layer situation, where the TS data were collected only from the lower layer, it is quite possible that incidental catches from the upper layer may contaminate the trawl sample. The ideal study location would only contain a single species with a narrow unimodal size distribution. The overall programme of target strength studies and the associated sampling should, however, be aimed at providing information on all life history stages of the species relevant to the population being surveyed. It is important to recognise that TS estimation requires precise sampling only of the stock component that contributes to the TS measurements, whereas acoustic abundance surveys require sampling which is aimed at providing information on the entire target population.