Music to oceanographers’ ears

Hydrophone
Photo courtesy of Kate Stafford

When it comes to the ocean, its depths are oftentimes less explored than outer space.

“There’s a tremendous amount we don’t know,” UW oceanographer Kate Stafford said. “I mean, it’s just staggering.”

In an effort to bridge the gap, Stafford and others have been using what are called haruphones or hydrophones—single underwater microphones. These hydrophones are weighted down by an anchor and placed on the ocean floor.

Via The Daily

On the other end of the hydrophone is a submerged floating device. When it’s time to find the hydrophone and retrieve the collected sounds, scientists send a signal to the hydrophone, which causes the floating device to detach from the anchor. While the anchor remains on the ocean floor, the hydrophone is dragged to the surface. 

Although the hydrophone remains underwater for a full year before scientists return to retrieve it, it does not record the entire time it is submerged. 

If the hydrophone were to record 365 full days of audio, its battery would die out too soon, and its hard drive would become too full. Only the first 20 minutes of each hour is recorded, Stafford said. 

Even then it’s too much data for researchers to listen to in its entirety, but Stafford said she ends up listening to a lot of it because “that’s the fun part.”

While Stafford said she has hydrophones in multiple places, the Bering Strait is the focal point. It’s the only gateway from the Pacific Ocean to the Arctic, meaning all migratory Pacific Arctic marine mammals pass through the strait twice a year. Some of these mammals include belugas, bowhead whales, walrus, and bearded seals.

“Additionally, it’s a good place to listen for subarctic species such as fin, humpback, and killer whales that seem to be spending more time north of Bering Strait,” Stafford said in an email to The Daily.

The reduction in ice levels has opened up more habitats for these subartic whales, and altered how long they stay in the Bering Strait. 

“So we’re looking at long-term trends in the detection of summer whales versus winter whales,” Stafford said. “We’re getting [the sounds] into October, and even November.”

Scientists like Stafford are also using the hydrophones to eavesdrop on bowhead whales. These particular whales have multiple “songs” that change from year to year and group to group, Stafford said, but no one knows why.

The hydrophones don’t just capture marine animal sounds. They also pick up moving icebergs, ocean currents, and ships.

While it was not the original intention of their research, scientists are now questioning how ship sounds affect whale communication.

“When ships go by underwater, the sound they radiate, we think, can make it more difficult for large whales to communicate,” Stafford said. “Shipping noise can mask the signals that whales or seals or other marine mammals produce underwater because they overlap on the same frequency range.”

Ships also have to use the Bering Strait, as well as the migratory animals, which makes it one of the best spots to see how ship sounds overlap with and affect migratory marine mammals. 

“The data are being used for quite a few studies, which is pretty exciting,” Stafford said. 

Research in the Arctic is especially difficult because it’s covered in ice most of the year, dark, and has poor weather, Stafford said. These can put limitations on most studies of marine mammals, but not for hydrophones. 

Spectograms, a visual picture of sound frequency versus time, are also useful to analyze audio. They essentially show the frequency, duration, and amplitude of any particular audio sample.

“This sort of sound doesn’t lull you to sleep,” Stafford said. “It can be quite noisy, and it’s not particularly melodious to the human ear.”

While working in the Arctic is a cold and expensive mission, it is also rewarding.

Stafford recalled a time when she was out on an Arctic ice edge with her team. The wind was so powerful, ice crystals from the frozen water were kicked into the air. Once the storm calmed, Stafford dropped her mic below the surface.

“It was like a concert hall,” she said. “The acoustics were perfect.”

The only noise in the water was a single bowhead song.

“It was just one of those moments where you want to be out on the ice all day long, listening,” Stafford said. “Often we won’t see any animals at all and yet, when you put the hydrophone down, you can hear all of them out there.”

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