Hearing footsteps under the ocean

Around 5 p.m. every evening the usual humdrum of the day starts to wind down at Birch Aquarium at Scripps. As the guests leave, voices and footsteps gradually fade away. A sealed underwater microphone switches on in a behind-the-scenes tank.

By night there is a cacophony of noises–munching, grabbing, calling or just moving. Simon Freeman wants nothing more than to hear this late night cacophony. He would like to figure out how many individuals are involved.

Freeman is a graduate student in the Marine Physical Laboratory at Scripps Institution of Oceanography. Working with Michael Buckingham, a professor of acoustics, he uses sound to monitor animal populations under the sea, noting that sound travels better and for longer distances in water than in air.

The idea of recording noises in the ocean is not new. Using a hydrophone, a specially designed microphone to record underwater sounds, scientists have been recording ocean noises since the 1940s. A pattern has emerged in the oceans: during the day, the noise level is low. After dark, the noise level increases.

At night, smaller marine animals take advantage of the fact that predators cannot see them. They come out of their hiding places to feed making incidental noises as they move about. These noises have features that are different than other sounds these animals make, such as mating calls. Freeman believes that incidental noise can be used to estimate the number and species of animals in a given area.

The technique could come just in time to document rapid changes in ecologically sensitive ocean areas such as coral reefs, which face threats from ocean acidification, pollution, and abrupt changes in ocean water temperature. One indication of the health of reefs is the number of animals present there. Annual surveys involving dives and underwater photography are conducted to monitor different aspects of coral reef health. The surveys are costly and require long planning and coordination.

Freeman’s research on ocean noise aims to develop a low-cost method to continuously monitor numbers of organisms on coral reefs. For his experiment, he is using hydrophones in shrimp tanks at Birch Aquarium at Scripps to record incidental sound.

“Shrimp have tough skeletons and when they walk on hard coral surfaces, it makes clearly recordable noise,” said Freeman.

In the tanks, there are a known number of shrimp so it is an ideal location to calibrate the noise to the number of animals. For the next year, Freeman will continue to listen to hours of underwater sound. He will be pondering questions such as whether ocean noise really reflects incidental noise by animals, how one can use recorded noise to estimate the number of individuals and whether particular species make noises that have distinct characteristics.

The process, however, is time-consuming. To get a two-minute recording of useful incidental noise, a researcher may have to record continuously for 48 hours. 

“But the low cost of the measuring devices and potential for having a continuous record of ecological parameters more than makes up for it,” said Freeman.

– Atreyee Bhattacharya has a Ph.D. in earth science from Harvard University and is a visiting student at the Geosciences Research Division at Scripps