In the common form of diel vertical migration, aquatic animals move upward toward the surface at night.
This daily movement is especially familiar among zooplankton, such as copepods, but it also occurs in fish, squid and other aquatic organisms. Animals usually rise around dusk, feed in shallow, productive water during darkness, and descend again around dawn. Darkness reduces the risk of being seen by visual predators while allowing access to surface food.
The pattern is driven mainly by changing light levels, although internal biological clocks, temperature, oxygen, salinity and predator activity can also influence it. “Diel” means a 24-hour cycle, not a season. The common nocturnal pattern should also be distinguished from reverse migration, in which animals rise during daylight. Diel vertical migration has consequences far beyond animal behavior: returning animals carry carbon to deeper water through respiration and sinking waste, helping power the ocean’s biological pump. The phenomenon was documented as early as 1817, and wartime sonar research later helped reveal its role in the deep scattering layer.