Schooling Behavior of Antarctic Krill Under Various Flow and Light Conditions

Antarctic krill (Euphausia superba) are an ecologically important species in the Southern Ocean food chain and swim in schools that extend several kilometers horizontally and hundreds of meters vertically. Krill schools are thought to increase swimming efficiency and enhance their collective response to environmental cues such as flow, light, and the presence of predators and prey. However, little is known about how krill schools respond to these cues. We built a novel annular flume having inner and outer diameters of 0.3 m and 1.2 m, respectively, a water depth of up to 0.3 m, and the capability of generating flows on the order of 1-100 mm s^-1. Flow was generated by rotating the inner cylinder and by submersible pumps and flow conditioners positioned along the outer wall and was characterized using particle image velocimetry (PIV). We conducted experiments at Palmer Station, Antarctica, where 700 krill were placed in the tank at a density of 5 krill L^-1 and were subjected to flow speeds of approximately 15 and 30 mm s^-1 at two different light levels. We filmed krill schools with a stereophotogrammetry system at 23.7 fps, and analyzed 3.5 minutes of footage per trial. Deep learning software and computer algorithms were used to measure krill positions in 3D space and time. Swimming speeds, school polarity, nearest neighbor distances, and nearest neighbor positions were analyzed.