Measurements of three-dimensional fish school velocities with an acoustic Doppler current profiler

Abstract

A method was developed to measure horizontal and vertical velocities of acoustic scattering layers using a multi-beam acoustic Doppler current profiler (ADCP). To determine three orthogonal velocity vectors (east, north, and vertical), it was required that the ADCP beams simultaneously insonified a coherently moving fish school in the same depth bin. Velocity vectors that satisfied these conditions were extracted from individual ping velocity estimates and ensemble-averaged to determine the average speeds and directions of fish aggregations. The results indicate that the ADCP can be a useful tool for observing fish and possibly zooplankton behavior in certain situations. Preliminary investigations include the quantification of horizontal and vertical migration patterns and possible vessel avoidance reaction of large schools of sardine in False Bay, South Africa. The method can be enhanced by refining equipment parameters such as water mode and depth bin length, correcting for rotational motion of the ADCP platform, and possibly by directly processing the radial velocity components from each beam.