npr:
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.
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