Towards estimates of zooplankton abundance and size distribution using sonar

To provide a system capable of insonifying zooplankton continuously and routinely at high speed, concurrently with other environmental parameters, we are developing a novel multifrequency system operating over a range of 200 kHz to 2 MHz, integrated within the existing IOSDL SeaSoar vehicle which currently carries a CTD, fluorometer, light and nitrate sensors. The frequency range will cover sizes of less than 1 mm to cms, and will cover the transition from Rayleigh scattering to geometric scattering for many animal classes. The acoustic sensor is being developed jointly by SOC (formerly IOSDL) and the University of Birmingham. It consists of 7 wide band transducers with centre frequencies spaced at half octave intervals. The transducer cluster is potted in an aluminium case and mounted in the nose of SeaSoar to insonify a volume of undisturbed water 6.5 m ahead of the vehicle.