The Acoustic Towfish is a system, which is towed in about 20 to 30 m water depth at the side of the ship. Its acoustic echo-sounder is pointing sideways, away from the ship to acoustically explore about the top 50 m of the water column. It allows to identify zooplankton density in the surface layers, which normally can not be detect by the ships build in echo-sounders, as these are sitting on the bottom of the ships hull at water depth of 6 to 8 meters. Also they have a so called blanking zone of up to 10 m, where the acoustic noise due to air bubbles is just to high to get any sensible data. This gab of the top 20 to 30 meter, which can not be sampled by the build in echo-sounders is fill by this Acoustic Towfish.
As with any echo-sounder the Towfish needs calibrating. This is an interesting operation, as the calibration booms have to be mounted onto the fish with their winches. Therefore you end up with the Towfish cable, 3 control cables for the three calibrations winches, the three lines for the calibrations sphere and the sphere itself. This setup has then to be deployed into the water without getting anything entangled:-)