The layers observed through the water column (vertical section of the sea) on echo sounders.

Echo sounders are measuring instruments that send out acoustic pulses in water and measure distances in terms of the time for the echo of the pulse to return.

SONAR is an acronym for "sound navigation ranging"; ASDIC is an acronym for "antisubmarine detection investigation committee".

The name refers to the way they scatter sound waves. The layers are typically composed of krill, midwater fish, and siphonophores (species which form complex free-swimming communities composed of numerous zooids of various kinds, or organic bodies or cells having locomotion, some of which act as floats or as swimming organs, others as feeding or nutritive zooids, and others as reproductive zooids).