Target tracking with a split-beam echo sounder

Abstract

The split-beam technique makes it possible to determine the precise location of a fish in the acoustic beam. This is a very useful feature, as it permits behavioural observations to be conducted without disturbing the fish. A split-beam echo sounder (Simrad ES400) interfaced with a computer has been used to track fish swimming through the acoustic beam. Tracks of reference targets guided through the beam of split-beam transducers on three research vessels and the transducer used for fish-behaviour experiments are presented with associated beam patterns. A "belt-fit" equation was used to represent the beam pattern measured in the tracking experiment. This was used to correct signal amplitudes for transducer directivity. In situ target-strength functions, representing the fish directivity pattern in the dorsal aspect, and three-dimensional plots of fish movement through the acoustic beam, are presented for three different gadoids.