Calibration, operation, and analysis of data from the Nortek Signature100 ADCP to estimate biomass and transport of Antarctic krill

In 2018, scientists from NOAA Southwest Fisheries, U.S. Antarctic Marine Living Resources (AMLR) program acquired a number of Nortek Signature100 ADCPs with integrated broadband echosounders for deployment on submerged moorings to make concurrent measurements of krill biomass and water flow. NOAA and Nortek collaborated to expand data collection abilities and to develop methods to calibrate the Sig100 echosounders. Calibration procedures for the Sig100 echosounder were based on echoes from a standard target (1.0” diameter, tungsten carbide sphere) located on beam-axis. Calibration experiments were conducted in NOAA’s technology development tank in San Diego, California. The calibrated, volume backscattering strength data from the Sig100 echosounder enable us to estimate biomass indices, from which biomass density may be estimated based on target scattering models and demographic data. In broadband mode, the echosounder operates over a frequency band from 68 to 114 kHz. Data from the pulse-compressed broadband echoes are binned into five frequency bands and recorded by the internal processor, and optionally are recorded as raw quadrature sampled voltage time series data to the Sig100’s internal memory device. NOAA/AMLR also operate the Sig100 in narrowband mode at two frequencies (70 and 120 kHz). These data enable comparison with vessel- and glider-based fisheries surveys. NOAA/AMLR deployed six submerged moorings equipped with Sig100s in an area north of the Antarctic Peninsula for a three-month period during 2018-2019 to estimate biomass (using Sig100 echosounder data) and transport (using Sig100 ADCP data) of Antarctic krill. Mooring buoys were located at 350 m depth to avoid icebergs, and the Sig100 beams point vertically upward. Sig100 echosounder data were recorded in 0.375 m range bins, and ADCP flow data were recorded in 5-m range bins, from the buoy to the water surface, at 6-second intervals. We implement methods to discriminate krill from other targets using two-frequency and multi-band echo classification. The Sig100 data produced time series of empirical measurements of krill biomass indices and water flow direction and magnitude over the water column. These data quantify krill biomass transport to inform models of prey availability to marine mammal and bird predators in the area.