What is a relatively dense aggregation of fishes, squid, and other mesopelagic organisms capable of reflecting a sonar pulse that resembles a false bottom in the ocean. Its position varies with the time of day.

Deep scattering layer, DSL

Explanation:

The deep scattering layer (DSL) also known as "deep sound layer", refers to an oceanic deep layer, more or less defined, that reflects sound and that is formed by a wide variety of animals. It is present in almost every ocean. It is produced by the presence of many organisms of different species that can disperse the sonar waves sent by a sonar equipment.

It was discovered through the ships´ sonar. Whenever they met a layer in which the sound hit, and was often confused with the bottom of the ocean. This is why the DSL is refered to as the "fake bottom".

These animals form layers of not less than 10 meters thick and might easily reach up to 50 meters thick. These layers are often found between 50 and 200 meters deep and can be seen going up and down every day according to the daily vertical migration.  

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