Deep-scattering Layer

deep-scattering layer, horizontal zone of living organisms, usually schools of fish, occurring below the surface in many ocean areas, so called because the layer scatters or reflects sound waves, causing echoes in depth sounders. Originally mistaken by some for the ocean bottom, the deep-scattering layer was later observed to rise toward the surface in the evening and to sink again at dawn, thus leading to a theory that it was composed of living organisms. Net tows through the layer and direct observation from submersibles confirmed the theory.