Evaluation of discrete target detection with an acoustic Doppler current profiler

Acoustic Doppler current profilers (ADCPs) use the signal scattered back from small suspended drifting particles, such as suspended sediments and planktonic species, to measure water current velocity. Additionally, ADCPs detect signals scattered from fish and other pelagic organisms, but these signals are generally treated as noise and rejected during data processing. However, these rejected signals can contain information on fish movement, which presents an opportunity to extend the application of ADCP technology as a tool for monitoring fish activity. We compare discrete target counts made using an ADCP with those of a split-beam echosounder in a region of high tidal currents in the Bay of Fundy. The comparison was achieved using a self-contained bottom-mounted frame equipped with both a 600-kHz Teledyne RD Instruments Workhorse ADCP and a 120-kHz BioSonics DTX Submersible split-beam echosounder system. The discrete targets were identified using a combination of signal correlation and volume backscatter (S-V) thresholds in the ADCP data and using a fish tracking algorithm in the split-beam echosounder data. The resulting ADCP discrete target counts agreed with fish tracking counts made from the co-located split-beam echosounder. Notably, discrete targets recorded by the ADCP could be differentiated from entrained air using a high signal correlation threshold. This method can expand the application of ADCP data for biophysical assessments for fisheries management and could be of particular use in rivers and tidal channels.