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.