Undersea Gliders Help Identify Ocean Sounds

The open ocean might appear to be a vast, mute void, but it is in fact a very noisy place. Below the surface, everything from grunt sculpins and rockfish to orcas and harbor porpoises emit sounds to communicate with one another. On the seafloor, urchins create a raspy din as they graze algae from rocks. Dense schools of herring keep in touch with one another via bouts of ultrasonic flatulence.

Sounds generally travel faster through water than air, and low-frequency sounds—such as those produced by some marine mammals and fishes—can propagate for considerable distances. The ocean is full of noise, as animals hunt, forage and chatter back and forth across their aqueous milieu.

At the Newport marina, OSU researchers upload survey coordinates into an autonomous glider prior to deployment

In recent years, researchers have found that this underwater soundscape is increasingly being muffled by human-made noise. Shipping traffic, oil and gas exploration, use of sonar by military vessels, and development in coastal and offshore waters—including renewable energy efforts such as the deployment and operation of wind, wave, and tidal energy converters—are among the loudest emitters, which can have negative impacts on marine ecosystems. To determine the exact nature and degree of these impacts, scientists rely on passive sound-recording devices to monitor how animals below the surface respond to “unnatural” noise.

Researchers at Oregon State University’s (OSU) Bioacoustics Lab are using these devices to determine the most effective way to listen in on these largely unseen animals. One such device is known as an autonomous glider, and the Aquarium recently helped the lab launch a pair of these gliders on a test run 30 miles west of Newport.

The gliders are remotely operated via satellite, said Selene Fregosi, an OSU Ph.D. student and NOAA scientist working with OSU’s Bioacoustics Lab. They’re propelled by internal buoyancy, which is controlled by an oil-filled bladder: drain the bladder of oil and the glider takes a nosedive into the depths; toward the bottom of the dive, pump oil into the bladder and the nose ascends.

“If there are any issues with the dives or ascents, the glider is programmed to come to the surface and send me a message through its satellite transmitter,” Fregosi said. “It’s kind of funny when the glider calls me on my cell, but it’s very useful in making sure that everything is going smoothly with its transect.”

A diagram showing the glider’s buoyancy-driven movement (Courtesy of Kongsberg Underwater Technology, Inc.)

With each dive and ascent, the glider makes lateral progress along its pre-determined transect. With no propellers, the progress is gradual—about half a knot per hour. But the upside of no mechanized propulsion is that the glider is nearly silent—perfect for recording the vocalizations of whales and dolphins. The acoustic data recorded by the gliders are stored on high-capacity memory cards similar to those used in digital cameras.

The gliders tested off Newport’s coast are scheduled for deployment in the Gulf of Mexico this month, where they will be used to record cetacean sounds in the waters affected by 2010’s Deepwater Horizon oil spill. Fregosi is piloting the gliders for this project—but she won’t be working anywhere near the Gulf.

“As the pilot, I stay in Oregon with a strong internet connection, working from my laptop—I’m unfortunately not out in the field,” said Fregosi. Despite the considerable distance, the gliders will respond to her instructions in real-time, so long as a reliable satellite signal is maintained.

“In addition to the gliders, this project will utilize two other types of passive acoustic monitoring systems: autonomous surface vehicles [similar to the gliders but operating at the surface] and bottom-moored hydrophones,” Fregosi said.

Findings from the project will help inform future efforts to monitor marine mammals via sound, both in the Gulf and closer to home, said Fregosi. One such effort takes place just west of the Aquarium, along the so-called Newport Hydrographic Line. For more than 30 years, a wide variety of data has been regularly collected on this offshore transect: seabird and marine mammal surveys, plankton trawls, water temperature and salinity, etc. Gliders equipped with acoustic recorders are used to collect snippets of cetacean vocalizations along the line, adding to this invaluable long-term dataset.

For additional information about the Bioacoustics Lab’s various projects, visit their blog.

After retrieval from a test voyage, OSU’s gliders return to shore aboard the Aquarium’s boat, Gracie Lynn
Information and photos provided by Oregon Coast Aquarium