The soundscape of deep-sea fish
We usually imagine the deep ocean visually, through memories of compelling underwater photography. But creatures of the sea live in a world of many dimensions, where sound is as important as sight, in a fluid medium that is an extraordinary transmitter of vibration.
Until recently, the sounds that deep-water fish made were inaudible to humans on the surface, due to the lack of an affordable recorder that could withstand the intense pressure of the water. When the technology arrived along with a grant funded by the MIT Sea Grant College Program, UMass Amherst Senior Research Fellow Rodney Rountree and his colleagues were the first to sink a listening device 2,237 feet below the surface of the North Atlantic.
Over 24 hours, they picked up an array of sounds: familiar fin, humpback, and pilot whale calls, but also 12 new unique and unidentified sounds: quacks, chirps, thumps, and grunts in the low frequencies that fish use.
Deep-sea fish are difficult to capture alive—their swim bladders explode under the pressure of surfacing—and they make behavioral sounds only in selected situations, calling for a mate from the dark seafloor, for example. So Rountree and his team are eavesdropping through “passive acoustics”: just listening, without interference, in the animals’ native habitat.
Studying the soundscapes of the deep sea is a new method marine biologists can use to monitor the health of the ocean and its species. “Noise can mask sound production so fish can’t communicate with each other,” says Rountree. He likens sound pollution to a human trying to call out to a friend across a busy city street. In an opaque environment that limits vision, like the sea floor, sound is terrifically important, and as shipping activity on the ocean rises, the risk increases that man-made noise could prevent deep-sea fish from communicating.
“We need to start thinking in terms of sound in the field of marine biology to understand the behavioral ecology of fish and other marine organisms,” says Rountree. “Bird biologists and frog biologists automatically listen for birds and frogs when they go into the field. We can do that with fish now.”