The acoustic buoy is a free floating sensor carrier with a scientific echo sounder as the main sensor. The unit can be positioned anywhere in the water column from surface to 1500 m depth with transducer pointing upward or downward or both. To enable precise abundance estimate of fish stock biomass knowledge of the acoustic TS (Target Strength) of individuals is essential. The Acoustic buoy can be positioned close to the fish and thus resolve dense concentrations to single fish. The unit has been used for establishing TS for several species such as blue whiting, cod, herring and even for krill in the Antarctic. The acoustic buoy permits silent environment measurements without disturbance from a surface vessel. The scientific sounders may thus record with much higher sensitivity compared to a ordinary installation onboard a ship. This instrument is ideal also for detecting and monitoring vertical migration of fish and plankton and species interaction. The buoy may take additional scientific payload such as camera, hydrophones and ADCP.
The sounder transducers are installed in upwards and/or downwards looking gimbals, keeping the transducers in a leveled position regardless of the tilt of the platform. The buoy is set up with standard instrumentation: Computer with ER60 software, 38 kHz split beam GPT and ES38DD transducer. A sensor unit with compass, inclinometer and depth sensor is installed on the transducer, giving actual level and direction of the transducer at all times. The instrument and battery containers are made of carbon fibre with end-caps and flanges made of titanium. The buoy has positive buoyancies, and is dropped with a weight connected to an acoustic release. The weight is dropped when activating the release and the buoy rises to the surface for retrieval.