Mesopelagic biological communities produce dusk choruses at Bermuda Rise

Biological evening chorusing from an unknown source has been reported in pelagic waters around the world, including the Eastern Indian Ocean, Western and Central Pacific Ocean, and Eastern Atlantic Ocean. Mesopelagic biological layers—consisting of aggregations of small fish and invertebrates—have been suggested as a possible source since they are common in the open ocean, and have a known diel vertical migration at dusk. Here we use directional acoustic sensing to localize an instance of the chorus in the Western Atlantic, which was found to be emanating from two distinct depths in close agreement with in situ echosounder measurements of the mesopelagic biological layers. Individual signals that comprise the chorus are isolated, and diel vertical migration of individual sources is observed, consistent with the known behavior of fauna in mesopelagic layers. During one of the evenings studied, echolocation clicks from delphinids which feed on these layers are observed diving toward the elevation angles where the chorus is present. These results indicate that mesopelagic biological layers are a major source of sound in the global ocean, producing choruses which likely act as beacons for predators and play a key role in pelagic ecosystem dynamics.