Schooling of Antarctic krill in a novel annular flume

Schooling is a key behavior of Antarctic krill, an ecologically important species in the Southern Ocean. Proposed benefits for school membership include increased hydrodynamic efficiency and improved awareness of external environmental signals, such as those created by predators, prey, or mates. Here we describe a novel annular flume used for exposing Antarctic krill schools at Palmer Station, Antarctica to various levels of flow, light, and odor. An annular design allows for continuous schooling without the animals encountering an up- or down-stream barrier as would occur in a linear flume. The tank inner and outer diameters are 0.3 m and 1.2 m, respectively. Variable flow in the range of 1-100 mm s^-1 is generated by rotating the inner cylinder and by submersible pumps and flow conditioners positioned along the outer wall and is characterized using particle image velocimetry (PIV). An overhead stereophotogrammetry system with near-infrared backlighting illumination beneath the flume allows three-dimensional measurements of krill positions at all ambient light levels. We report school polarity, swimming speed, and nearest neighbor distance results from experiments in which 700 krill were subjected to flow speeds of approximately 15 and 30 mm s^-1 at two different light levels.