The experiments described in this paper relate known target configurations
under controlled conditions to acoustic characteristics of multiple moving
fish. We wanted to further our understanding of the interactions between
targets and the effects these interactions have on the measurement of the
number of salmon migrating in rivers. Multiple targets in various
configurations were passed through a horizontally oriented 4° × 10° beam from a split-beam echo sounder. The effects on measurements of
target strength, detection probability and target location in the beam are
presented. The observed target strength was not dependent on target
velocity. There was a reduction in target detection due to the single-target
selection criteria implemented by the hydroacoustic system. We mimicked the
conditions in a river where a close range fish target may modify the beam
geometry allowing detection of previously undetected targets. We
demonstrated some of the effects resulting from moving targets into radial
alignment and we demonstrated shadowing conditions that can cause extinction
of target echoes.