Spatial distribution of Antarctic krill (Euphausia superba) density in the Krill Research Zone of the Ross Sea Region Marine Protected Area, Antarctica

This report provides the first acoustic observation of Antarctic krill (Euphausia superba) density distribution in the Krill Research Zone (KRZ) of the Ross Sea Region Marine Protected Area (RSR MPA). We investigated the horizontal and vertical distribution of krill density by developmental stage (juvenile, sub-adult, and adult) using multi-frequencies (38, 120, and 200 kHz) and examined associated swarm characteristics. Juvenile krill dominated higher-latitude stations near the ice-edge, whereas sub-adults and adults were primarily found at lower latitudes. Spatially, krill density was notably greater in the eastern portion of the KRZ. Vertically, approximately 90% of krill swarms occupied the upper 100 m of the water column, while ~8%, mainly adults, were located between 150–250 m depth. Significant differences among developmental stages were found in swarm depth and packing concentration (p < 0.01), with older stages occurring deeper and packing concentration highest in sub-adults, intermediate in juveniles, and lowest in adults. Total krill volumetric density showed significant negative correlations with water temperature and distance from the ice edge (p < 0.05). Sub-adult and adult volumetric densities negatively correlated with temperature and salinity but positively correlated with dissolved oxygen and food availability (p < 0.05). These findings provide a baseline for krill distribution in the KRZ, informing management of the RSR MPA. The results underscore the importance of regular acoustic monitoring in remote Antarctic regions to detect changes in krill populations under environmental change.