Acoustic estimation of Antarctic Krill Biomass using two- and three-frequency methods in the Krill Research Zone of the Ross Sea Region Marine Protected Area

This report provides the first acoustic estimate of Antarctic krill (Euphausia superba) biomass within the Ross Sea Region Marine Protected Area’s Krill Research Zone (KRZ). We compared two-frequency (38 and 120 kHz) and three-frequency (38, 120, and 200 kHz) acoustic methods to assess krill distribution and biomass. Both acoustic approaches yielded similar biomass estimates and identified clear spatial heterogeneity, with significantly higher krill densities concentrated in the eastern part of the KRZ. Approximately 90% of krill swarmss occurred within the upper 100 m of the water column, indicating a strong preference for near-surface habitats. Estimated krill density ranged from 0 to 285 g m⁻² across both methods, with negligible differences in spatial distribution patterns. The mean estimated krill biomass was 0.5974 million tonnes (two-frequency method) and 0.5932 million tonnes (three-frequency method), reflecting a minor difference of approximately 4,200 tonnes (0.7%) between methods. The 95% confidence interval for krill biomass ranged broadly from 0 to 1.33 million tonnes. These baseline results emphasize the need for continued acoustic and biological monitoring to assess interannual variability, support ecosystem-based management by CCAMLR, and provide foundational data necessary for any future research-oriented krill fisheries in this ecologically sensitive marine protected area.