Deep scattering layer

[4] The phantom bottom is caused by the sonar misinterpreting as the ocean floor a layer of small seagoing creatures that congregate between 1,000 and 1,500 feet (300 and 460 m) below the surface.

[6] Most mesopelagic fishes are small filter feeders which ascend at night to feed in the nutrient rich waters of the epipelagic zone.

They ascend at night into the shallow epipelagic zone, often following similar migrations of zooplankton, and return to the mesopelagic depths for safety when there is daylight.

[11] Lanternfish are among the most widely distributed, populous, and diverse of all vertebrates, playing an important ecological role as prey for larger organisms.

The previous estimated global biomass of lanternfish was 550–660 million tonnes, about six times the annual tonnage captured worldwide by fisheries.

Static image of a sonar scan. The backscattered signal (green) above the bottom is likely the deep scattering layer. [ 1 ]
The swim bladder (marked here as S and S' ) of Alburnoides bipunctatus . The swim bladders of large numbers of mesopelagic fishes cause sonar waves to be reflected in a recognisable layer
Lanternfish account for as much as 65 percent of all deep sea fish biomass and are largely responsible for the deep scattering layer of the world's oceans.
Time lapse video of a 3-D mapping of water column sonar data by the NOAA research ship Okeanos Explorer in the North Atlantic Ocean [ 1 ]