Fish rush hour is causing a "hum" in the Pacific Ocean, say scientists Getty Images

Astronauts may have heard strange outer-space music decades ago, but scientists have also been puzzling for years over an intriguing "hum" or buzz in the Pacific Ocean that they're now linking to the massive movements of fish populations.

The mystery began with hydrophones as marine scientists dropped these underwater microphones over ships or attached them to buoys to listen to the songs of humpback whales or the clicking of dolphins.

But they also heard something unusual and inexplicable. The "rushing" sound, a bit like traffic, was faint but continuous at certain times of day, and just a few decibels above the background level. It was high for the call of a whale and too continuous to be the signals of other mammals, reports National Public Radio.

It's "as if you're sitting on an airplane and it's humming, buzzing," explained Simone Baumann-Pickering, a marine biologist at the Scripps Institution of Oceanography at the University of California in San Diego.

Huge numbers of small fish, crustaceans and squid tend to hide in deep water during the day, then rise to feed nearer the surface in the food-rich mesopelagic zone. Scientists tracking hydrophone sounds in the Pacific eventually determined that the humming accompanied the daily rise and fall of the fish migration.

The sound starts after sunset and continues for a couple of hours, then repeats at dawn.

The fish may be "truly, actively communicating — potentially to initiate migration," saying, in effect, that "it's time to go," Baumann-Pickering told a meeting of ocean scientists in New Orleans.

There could be another explanation, she acknowledged.

"It's known that some fish are considered to be farting. They emit gas as they change depths in the water column," she explained.

It could also simply be the sound of the daily travels of billions of fish, 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.

As for the sound, "we're just scratching the surface in terms of understanding how important" it is in the ocean, she adds.

More about the Pacific Ocean