Once in Antarctica, the whales feed mainly on krill. The krills gather to form large swarms that migrate vertically during the day. While they stay deep during the day to avoid predators, they come close to the surface at night. Krills have also been observed up to 3,000 meters deep. It is an important source of food for many marine mammals (whales, seals, leopard seals) but also squid, Antarctic fish, penguins, albatrosses and other bird species.