Monitoring Antarctic krill (Euphausia superba) distribution in the Southern Ocean: environmental DNA (eDNA) adds to the toolbox

Leonie Suter^1,2*Alicia Burns^3,4Sophie Bestley^2,5Jez Bird²Madeleine Julie Brasier^1,2Martin Cox^1,5Derek Hamer⁶Olivia J Johnson²So Kawaguchi^1,2,5Rob King^1,5Andreas Klocker⁷Jessica Melvin²Christine K Weldrick^2,5Simon Wotherspoon¹Ben Raymond^1,5

• ¹Department of Climate Change, Energy, the Environment and Water, Australian Antarctic Division, Hobart, Australia
• ²Institute for Marine and Antarctic Studies, College of Sciences and Engineering, University of Tasmania, Hobart, Tasmania, Australia
• ³Cluster of Excellence Science of Intelligence, Technical University Berlin, Berlin, Berlin, Germany
• ⁴Faculty of Life Sciences, Humboldt University of Berlin, Berlin, Berlin, Germany
• ⁵Australian Antarctic Program Partnership, Institute of Marine and Antarctic Studies, University of Tasmania, Hobart, Australia
• ⁶Palmyra, Little Swanport, Australia
• ⁷NORCE Norwegian Research Centre, Bjerknes Centre for Climate Research, Bergen, Norway

Antarctic krill (Euphausia superba Dana) is a key species of the Southern Ocean ecosystem, immensely abundant and targeted by the krill fishery. For their sustainable management, krill distribution and biomass estimates are required, typically achieved through acoustic-trawl surveys. We explore how krill environmental DNA (eDNA) can contribute to our understanding or Antarctic krill habitat and distribution. We collected eDNA samples by filtering five litres of seawater per sample in the East Antarctic Southern Ocean from the surface (5 m depth) and seafloor (381 - 4422 m depth, total n = 110). We used quantitative PCR to measure Antarctic krill eDNA abundance and age, and eDNA metabarcoding to detect any krill species. This eDNA data was compared to acoustic, visual and trawl detections of Antarctic krill. Antarctic krill eDNA was common in surface samples and largely overlapped with visual and trawl detections. Highest eDNA concentrations were detected above krill swarms, with concentrations declining with increasing distance from swarms. Near recent eDNA sampling locations, krill swarms were more likely acoustically detected than near old eDNA sampling locations. Antarctic krill detections were less common in seafloor locations, and detections were concentrated in the continental slope area to the south of the survey area, both for visual detections and for recent eDNA detections. Both methods detected Antarctic krill at great depths (recent eDNA: 4300 m; visual: 3080 m). In both eDNA and trawl data, Antarctic krill was the dominant krill species, followed by Thysanoessa macrura G.O. Sars, which was particularly abundant in larval stages throughout the survey area, including at Antarctic krill swarm locations. We recommend the inclusion of eDNA data for Antarctic krill distribution estimates and understanding of habitat use, particularly in difficult-to-access areas, such as under ice or benthic habitats.