The effects of flow, light and group density on Antarctic krill (Euphausia superba) schooling behavior

Antarctic krill (Euphausia superba) are a key species in the Southern Ocean food web and form various aggregation types, from diffuse swarms to organized schools. However, little is known about how environmental variables such as flow and light affect the organization of these aggregations. Here, we investigate the effects of light, flow and group density on the group-level behavior and organization of Antarctic krill in an annular flume. The experimental setup comprises a rotating inner drum and pumps with flow conditioners to adjust flow speed, overhead lighting with variable intensity and infrared backlighting for filming. An overhead camera measures group organization and a stereophotogrammetry system records three-dimensional krill swimming trajectories. We conducted experiments at Palmer Station, Antarctica, varying krill group density (1-19 krill l-1), flow speed (no flow, low flow and high flow) and light level (light versus dark). Without flow and regardless of density, krill were significantly more organized in the light than in the dark, indicating the important role of vision in school formation. However, regardless of density, high flow produced similar levels of increased group organization in both the light and the dark, indicating that hydrodynamic cues may be sufficient to organize a krill school via positive rheotaxis. Density strongly affected group organization, with maximum organization seen at group densities of 1 and 7-9 krill l-1 irrespective of light or flow. These findings illuminate environmental effects on krill collective behavior and may lead to a better understanding of field acoustic data and improved krill biomass estimates.