Throughout the world ocean, animals congregate at certain depths, forming layers that can be hundreds of meters thick and may extend horizontally for dozens or even hundreds of kilometers. Because these dense layers of animals reflect sound waves, they are sometimes called “sound-scattering layers” or “deep scattering layers” (though they can occur near the sea surface). A new paper in Limnology and Oceanography shows that, rather than consisting of a random mixture of animals, these layers contain discrete groups or “schools” of squids, fishes, and crustaceans.

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