The Effects of Flow, Light, and Aggregation Density on Antarctic Krill School Behavior

Antarctic krill (Euphausia superba) are a key species in the Southern Ocean food web. This species forms a variety of aggregation types (e.g. massive schools, diffuse swarms) which may enhance swimming efficiency or improve awareness of predators, prey, or mates. However, little is known about how krill aggregations respond to environmental variables. Here we describe a novel annular flume designed to test the response of Antarctic krill schools of different densities to various light and flow levels in the laboratory. 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 (bright light vs complete darkness). An overhead camera showed that krill schools were most organized at a density of around 9 krill L^-1 and, in the absence of flow, were more organized in the light as compared to the dark at all school densities. Further, flow generally organized the krill schools via rheotaxis, especially when light was absent. A synchronized high magnification stereophotogrammetry system produced corresponding swimming speed and nearest neighbor distance distributions, revealing that the more organized krill swam faster. These results indicate the relative importance of group density, vision, and rheotaxis in the formation and maintenance of krill schools. The facility developed here also can be used to test other schooling organisms such as fish.