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Mystery Ocean Hum May be Migration Signal, or Fish Farting

Communities of deep-sea marine life cause a low-frequency humming in the ocean, as the creatures swim to and from the surface to feed.

Vast communities of migrating deep-sea marine life are the culprits behind a mysterious, low-frequency humming sound in the ocean, made as the creatures swim to and from the surface at feeding time.

The discovery, made by University of California, San Diego assistant research biologist Simone Baumann-Pickering, answers a long-standing question. The source of the hum has for years vexed marine biologists, as NPR reports. They knew the sound wasn't consistent with whale calls or other marine mammals, such as dolphins, communicating.

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Now, thanks to high-sensitivity undersea audio recordings, Baumann-Pickering says it's animals such as fish, jellies, shrimp, and squid living in what's known as the ocean's mesopelagic zone – a range 200 to 1000 meters (660 to 3300 feet) below the surface – that are behind the sound.

Creatures in the mesopelagic neighborhood live deep down, in a dark world where the sun barely shines and there's not exactly a bounty of food. So each night, with the safety of darkness, they venture up to the surface where food is more plentiful.

And when they head up top (or back down) the hum -- about 3 to 6 decibels louder than ocean background noise -- kicks in.

"It's not that loud," Baumann-Pickering said in a statement. "It sounds like a buzzing or humming, and that goes on for an hour to two hours, depending on the day." (Check out a sound clip of the humming here.)

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The purpose behind the sound is still an open question. Baumann-Pickering said it could be a signal to the entire group to head up to the surface or back down.

While it's neat to think that such communication could be happening among the animals, there could also be a less high-minded reason for the hum. It turns out the creatures might just be passing gas, as their swim bladders regulate their buoyancy.

"It's known that some fish are considered to be farting," Baumann-Pickering told NPR, "that they emit gas as they change depths in the water column."

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If the denizens of the mesopelagic are engaging in communication of some kind, then learning more about the messages being conveyed and which specific animals are doing the conveying would help scientists come to a better understanding of the ecosystem they inhabit, according to Baumann-Pickering.

It could also tell researchers more about the predators that feed on the mesopelagic animals -- if the hunters are listening in, then the hum may tip them off that food is nearby.

Baumann-Pickering presented her findings at the Ocean Sciences Meeting being held in New Orleans.

Cuttlefish are among the creatures that call mesopelagic depths home.

This is the James Clark Ross, a ship run by the British Antarctic Survey that carried Sue Scott and other researchers on a journey to Tristan da Cunha, a remote island and archipelago in the South Atlantic.

The journey was funded in part by the Pew Charitable Trusts. Over the past decade, Scott has made dozens of dives in the rough water surrounding the island and helped chronicle the unique life there. She's based in northwestern Scotland but finds herself repeatedly drawn to the island — this was her eighth trip — and is one of the few experts on the sea life there. Until now, nobody had seen what lurks just beyond the range of scuba divers, at a depth of about 150 to 300 meters (492 to 984 feet) beneath the ocean's surface.

This is the larvae of a rock lobster (Jasus Tristani) which, at this life stage, is called a puerulus. When it was first found, few of the biologists on board knew what it was.

The seas slugs were collected from the ocean floor near the island of Gough, which is part of the Tristan archipelago.

This little guy was collected by a seafloor trawl near Gough, which is part of the Tristan archipelago. Like all hermit crabs, it uses the shells of other animals in which to live.

These cup corals were found in large numbers in the waters near Tristan da Cunha, at depths between 150 to 300 meters (492 to 984 feet).

The cup corals appear to thrive in the waters beyond the reach of divers, making due with the scant light that penetrates.

This is the island of Tristan da Cunha, with the settlement — known as Edinburgh of the Seven Seas — on the right. On the left is a volcano that erupted in 1961, and the scar from a recent rock fall. The island has a population of about 260 residents.

Recently, islanders built a replica of a traditional Tristan house made of volcanic rock with a roof thatched from New Zealand flax. The house is nestled between lava flows from the island's 1961 eruption.

This larval eel head was photographed by a mid-water trawl, suspended above the seafloor off of Tristan.

After using sonar imaging to search for flat areas around the seafloor, the camera system is lowered to 150 to 300 meters (492 to 984 feet).