You might expect the middle of the Pacific Ocean to be a pretty quiet place, especially a thousand feet down. But it turns out that huge parts of the ocean are humming.
Scientists have puzzled over the source of the sound for several years. Now, a marine biologist reporting Monday at a meeting of ocean scientists in New Orleans says she thinks her team may have figured it out.
The discovery started with hydrophones. Marine scientists listen to the deep ocean by dropping these underwater microphones over the side of ships, or by putting them on buoys. Usually they hear what sounds like male humpback whales calling to each other at mating time, or the clicking signals of dolphins and other marine mammals.
But a few years ago, those hydrophones picked up something weird out in the Pacific. The puzzling sound was faint, but continuous at certain times of day — just a few decibels above the background level — and definitely different from the normal sound of the ocean. It was 300 hertz and above — high for the call of a whale, and too continuous to be the signals of other marine mammals.
According to Simone Baumann-Pickering, a marine biologist at the Scripps Institution of Oceanography at the University of California, San Diego, the nature of the sound was "more as if you're sitting on an airplane and it's humming, buzzing." The sound starts after the sun sets, she says, and goes on for a couple of hours, then stops. The same thing happens at dawn.
Now, biologists knew that huge clouds of small fish and crustaceans and squid tend to hide in the dark, deep water during the day, and rise up nearer the surface to feed at night. This happens in all oceans in the mesopelagic zone, a fish-rich area of little light that stretches from about 660 feet beneath the sea's surface to depths of around 3,300 feet.
It took hydrophones in the Pacific to reveal that the hum actually accompanies that daily rise and fall of the fish migration.
Why the noise? Scientists can only speculate. It could be, says Baumann-Pickering, that the creatures "are truly, actively communicating — potentially to initiate migration." In other words, maybe the buzz is just a signal that "it's time to go," she says.
But there's another more mundane possibility.
"It's known that some fish are considered to be farting," says Baumann-Pickering, "that they emit gas as they change depths in the water column." The gas comes from a swim bladder inside the fish that controls its buoyancy.
In either case, billions of fish may be jetting up and down in the ocean every day, and making it hum. If so, it would likely be the largest migration of vertebrate animals on the planet, Baumann-Pickering says. She calculates that the weight of the fish amounts to some 10 billion tons.
At the New Orleans meeting sponsored by the American Geophysical Union, Baumann-Pickering told colleagues the sound raises an interesting question. Why would the fish do this if, as you might expect, the noise would have the effect of a dinner bell, attracting large predators?
Nobody knows, she says. "We're just scratching the surface in terms of understanding how important sound is" in the ocean.
AUDIE CORNISH, HOST:
You might expect the middle of the Pacific Ocean to be a pretty quiet place, especially a thousand feet down, but huge parts of the ocean are humming. Scientists have been trying to figure out what it is for several years. And as NPR's Christopher Joyce reports, they now have their answer, and it's a real surprise.
CHRISTOPHER JOYCE, BYLINE: We usually hear the ocean when it meets the land where waves roll up onto the beach. But marine scientists like to listen to the deep ocean where they drop hydrophones overboard - underwater microphones - to listen for things like humpback whales calling to each other.
(SOUNDBITE OF WHALE CALL)
JOYCE: But a few years ago, those hydrophones picked up something weird. It was faint but different from the normal background sound.
SIMONE BAUMANN-PICKERING: It's definitely more a - as if you're sitting on an airplane, and it's humming, buzzing.
JOYCE: Simone Baumann-Pickering is a biologist at the Scripps Institute of Oceanography in California. She says the sound starts as the sun sets, stops overnight then picks up again for two hours at dawn. Now, biologists know that huge clouds of small fish hide in the dark, deep water during the day and then rise up near the surface to feed at night, and the hydrophones revealed that the fish make the ocean hum when the rise and fall. It could be they're signaling each other that it's time to move, or...
BAUMANN-PICKERING: It could be - for example, it's known that some fish are considered to be farting - that they emit gas as they change in the depths in the water column.
JOYCE: The gas comes from a swim bladder inside the fish that controls its buoyancy. Imagine billions of fish literally jetting up and down in the ocean every day. This would have to be the largest migration on the planet.
BAUMANN-PICKERING: Oh, for sure, absolutely.
JOYCE: Baumann-Pickering described her research at an ocean science meeting today. She says it raises an interesting question. Could all this noise be like a dinner bell attracting large predators to these clouds of small fish? Nobody knows, she says. Christopher Joyce, NPR News. Transcript provided by NPR, Copyright NPR.