Diel vertical migration
The daily movement of fish and plankton from the deep sea up to the surface at night and back down at dawn.
Also: DVM
Every night, huge numbers of lanternfish, other small fish and zooplankton rise from depths of several hundred metres to feed near the surface, where food is plentiful. At dawn they sink again into the dark, where they are safer from predators that hunt by sight. Measured by the weight of animals moving, it is often called the largest migration on Earth. Sonar operators in the Second World War saw it as a false sea floor that rose at night, called the deep scattering layer.