ADCP Velocity Errors from Pelagic Fish Schooling around Equatorial Moorings*

Abstract

Beginning in 1990, PROTEUS (profile telemetry of upper ocean currents) moorings have been deployed in the equatorial Pacific with the ability to transmit ocean current measurements to shore in real time via satellite. The surface moorings were equipped with 153.6-kHz RD Instruments acoustic Doppler current profilers (ADCPs), along with mechanical current meters (MCMs) at six or seven depths. At times, large bias errors were found in the ADCP velocities relative to MCM velocities, due to the reflection of acoustic energy from fish in the vicinity of the moorings. An algorithm was subsequently developed and added to the ADCP firmware in an attempt to identify and reject fish-affected data before ensemble averaging of individual pings. The algorithm rejected some fish-biased data, but large velocity errors still occurred in the ADCP averages. A technique utilizing empirical orthogonal functions of ADCP?MCM speed differences was developed to correct the ADCP velocities after mooring recovery. The corrected daily averaged time series were found accurate to within ±5 cm s?1 at the depths where significant fish bias occurred.