U.S. Navy sonar linked diel vertical migration to what ocean sonar feature?

U.S. Navy sonar linked diel vertical migration to the deep scattering layer, an oceanic band of animals that reflects sound and can resemble a second seafloor. The layer is also called the sound-scattering layer or, informally, the phantom bottom.

During World War II, sonar operators noticed an apparent seabed hundreds of metres below the real one. It moved upward at night and downward during the day, an observation that initially puzzled naval scientists. The changing depth matched diel vertical migration, the daily movement of marine organisms through the water column.

The layer is produced by huge numbers of organisms, especially small mesopelagic fish such as lanternfish, along with squid and other animals. Fish swim bladders are particularly effective at reflecting sonar pulses. At dusk, many organisms rise into shallower water to feed on plankton; by daylight, they descend into darker depths where they are less exposed to predators.

The feature is not a solid geological boundary. Its position varies with light, weather, moonlight, food availability, and local ocean conditions. The discovery helped reveal how much living biomass occupies the ocean’s middle depths.