Biomass Density and Transport of Antarctic Krill Measured Using Echosounders and ADCPs Aboard Autonomous Gliders and Moorings

In 2018, the U.S. Antarctic Marine Living Resources (AMLR) Program managed by the Antarctic Ecosystem Research Division of NOAA Southwest Fisheries transitioned from ship-based surveys to an autonomous observing system. AMLR now uses long-duration underwater gliders and an array of moorings to estimate biomass and transport of Antarctic krill near the Antarctic Peninsula. We deployed a combined mooring array and flew glider surveys where krill-dependent penguins, seals, whales, and other predators forage. All instruments were deployed for a three-month period during austral summer 2018-2019. The two, deep (1000 m), Slocum G3 gliders were each equipped with extended primary lithium batteries, a pumped Sea-Bird CTD, a WET Labs ECO puck, an Aanderaa oxygen optode, and a three-frequency (38, 67, and 125 kHz) ASL Acoustic Zooplankton Fish Profiler (AZFP).The six moorings were submerged (350 m or as depth allowed) to avoid icebergs and were equipped with upward-looking Nortek Signature100 ADCPs with integrated broadband echosounders (Sig100). Four moorings were located in canyons, one was on the continental slope, and one was on the shelf. Gliders provided vertical profiles of oceanographic and acoustic data that resulted in estimates of overall krill biomass density for each of the survey areas. Moorings provided time series of empirical measurements of krill biomass indices and water flow direction and magnitude over the water column. We quantified the seasonal variability in krill and fish in the vicinity off Cape Sherriff an important reproductive area for seals and penguins. We linked these estimates to patterns of foraging by land-based predators. Together these data quantify krill biomass transport to inform models of prey availability to marine mammal and bird predators in the area.