Antarctic krill (Euphausia superba) swarm structure and biomass distribution around the South Orkney Islands (Southern Ocean)

Antarctic krill swarm structure, biomass, and distribution to the northwest of the South Orkney Islands, Southern Ocean, were investigated using a combination of acoustic and scientific net trawl data. Krill is fished commercially in this region and is a major prey item for many higher predator species, but little is known of the temporal-spatial variation in its distribution, biomass, and behaviour. During a multi-disciplinary cruise in January-February 2016, krill swarms and biomass density were measured across different scaled acoustic surveys (~500, 3000, 8500 and 28,000 km2). Krill swarm metrics, including abundance, area, length, thickness, 2D roughness and depth were quantified and related to several environmental factors. Krill was distributed widely along the northern shelf-break, but biomass densities varied between the different spatial scaled surveys typically within a ~10 d period (~1 to 280 g m-2). Local canyon regions, which are often encompassed by the fishery, were hotspots of krill biomass density and had the greatest number of swarms. Chlorophyll a fluorescence was often elevated in these canyon regions and this parameter was also the most consistent predictor of swarm depth and area, although variation in swarm metrics was high. Swarms in the canyon regions were significantly (P<0.01) smaller, deeper, rougher-shaped, and closer together than swarms in more off-shelf regions. Our study suggests that local bathymetry and oceanographic processes act to concentrate krill in canyons. Furthermore, local differences in swarm structure suggest that krill undertake a behavioural response to the small-scale environmental conditions, particularly food availability, in the canyon regions. This regional heterogeneity will influence local predator-prey interactions as well as the spatial distribution of the krill fishery. This study contributes important information to underpin robust ecosystem-based management in this region.