Distribution and biomass estimation of Antarctic krill (Euphausia superba) off the South Orkney Islands during 2011-2020

Antarctic krill is a key species in the Southern Ocean and subject to the most extensive fishery in the Antarctic. The Norwegian Institute of Marine Research has conducted acoustic-trawl monitoring of krill off the South Orkneys annually since 2011 in collaboration with the krill fishing industry. Average krill biomass within the 60000 km² survey area ranged from 1.4 to 7.8 million tonnes in the period 2011-2020, strongly supporting that this is among the regions in the Scotia Sea with consistently highest krill densities. There were no significant (p approximate to 0.18, non-parametric Mann-Kendall test) monotonic trends in estimated krill biomass over the 10 years. The highest krill densities were associated with the shelf edge and submarine canyons on the north side of the South Orkneys. Our comparison with the CCAMLR 9.3% reference exploitation rate suggests that management of the krill fishery in the South Orkneys region is precautionary. The monitoring is run on fishing vessels, so e.g. acoustic frequencies applied could not always be in compliance with the standard CCAMLR methodology. Estimated deviance in krill backscatter when comparing 38 kHz to the standard 120 kHz ranged from -1.1% to 12.8%. Our results show that industry-based surveys are cost-efficient approaches to high-quality monitoring of krill.