Deep scattering layer

A layer of marine organisms that strongly reflects sound, creating a sonar-visible band that typically rises toward the surface at night and descends during the day.

Diel vertical migration: The regular daily movement of organisms between deeper water by day and shallower water at night. Its synchronized nightly ascent and daytime descent shifts the scattering layer through the water column.
Mesopelagic fish: Fishes that live primarily in the ocean’s mesopelagic zone. They are among the abundant animals responsible for scattering sound in the layer.
Acoustic survey: A method that uses sound measurements to estimate the distribution or abundance of underwater organisms. Researchers map and quantify scattering layers with calibrated acoustic surveys.
Mesopelagic biomass: The total mass of organisms living in the ocean’s mesopelagic zone. Estimates vary widely because acoustic returns do not translate simply into organism counts or mass.
Swim bladder: A gas-filled organ that helps many bony fishes control buoyancy. Gas-filled swim bladders reflect sonar strongly and make many fish conspicuous within the layer.
Zooplankton: Small drifting animals and animal-like organisms that live in the water column. Dense aggregations of zooplankton contribute to the layer’s acoustic returns.
Marine snow: Falling particles of organic matter and other material in the ocean water column. Animals migrating through the layer transport carbon downward through feeding and waste production.
Target strength: A measure of how strongly an individual object reflects sound back toward a sonar. Species, size, orientation, and anatomy alter the signal used to estimate layer abundance.
Mesopelagic zone: The ocean depth zone extending roughly from 200 to 1,000 meters, where sunlight is faint. Much of the layer occupies this dim zone during daylight.
Krill: Small, shrimp-like crustaceans that form large swarms in marine waters. Krill and related crustaceans can form dense scattering aggregations.

Linked from 20 pages