ESTIMATION OF ZOOPLANKTON ABUNDANCE FROM SHIP-BORNE ADCP BACKSCATTER

The backscattered signal intensity from a shipborne RD Instruments 150 kHz Acoustic Doppler Current Profiler is compared with zooplankton abundance during surveys around the Indian Ocean island of Aldabra. Significant correlation is found between the range-corrected backscatter summed from the surface to a depth of 200 m and the biomass collected by a net haul over the same vertical range at the same station. Biomass is expressed as mg C m-3 and is obtained by using a modified displacement volume method. Unlike previous work by FLAGG and SMITH (1989, Deep-Sea Research, 36, 455-474; 1989, Proceedings of OCEAN '89, Marine Technology Society and I.E.E.E.) the ADCP was unmodified. This indicates that, until such modifications and calibrations as they suggest are carried out, data from existing instruments are still worth investigating. Problems involved in the determination of flow noise inherent in shipborne ADCP backscatter measurement are discussed. Care must be taken if backscatter is studied when the ship is steaming. Nevertheless the application of the ADCP to combined studies of flow fields and biological productivity offers promise of continuous along-track estimates of zooplankton biomass when supplemented by spot calibrations.