An assessment of the utility of an acoustic Doppler current profiler for biomass estimation

Acoustic Dopplerd current profilers (ADCPs) are being used increasingly to derive estimates of zooplankton and micronekton abundance. The absence of a practical means for users to perform ADCP calibration, however, makes the quantitative value of this approach questionable. In an attemptto evaluate ADCP-derived volume backscattering strength (Sv) data, and hence to assess the utility of such measurements for biomass estimation, a regularly calibrated Simrad EK500 scientific echo-sounder (operating at 38, 120 and 200 kHz) and an RDI narrow-band ADCP (153.6 kHz) were similarly configured and run in synchrony on a transect in the Southern Ocean. Data were collected by both instruments from congruent depth (4 m) and time (2 min) bins in order to allow direct comparison of numerous discrete values without the need for further signal averaging. Echoes were recorded from the Antarctic krill, Euphausia superba,in deep-ocean and on-shelf locations during day and night. ADCP-derived volume back scattering strength data from shallow, evenly distributed krill targets were well correlated with equivalent data from the EK500 (r2>0.98), and the offsets between instruments conformed to those predicted for their respective operating frequencies by a theoretical model of sound scattering by krill (H0: Sv 153.6 kHz=Sv 120 kHz+2.3 dB, t0.05(2),155=1.98, t=0.74, P=0.46). Data from deeper, more irregularly distributed targets differed significantly (P<0.001). We conclude that under some ideal, but probably rare, circumstances data from the ADCP can be used to derive biomass estimates. The numerous uncertainties surrounding ADCP calibration and the current practical impossibility for users to monitor system performance should, however, preclude these instruments from being used as a matter of course to determine abundance estimates, a task that we believe should remain firmly within the domain of a well calibrated scientific echo sounder.