Published online 9 November 2009 | Nature | doi:10.1038/news.2009.1069

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Whale-song recording goes deep

Underwater glider eavesdrops on cetacean communication.

A better way of monitoring marine mammalsApplied Physics Laboratory, University of Washington

Silently slipping to 1,000 metres below the ocean surface, an undersea glider equipped with a recording device is cruising off Hawaii to capture unprecedented detail on the sounds made by whales.

The experiment represents the first time that an acoustic-equipped glider has been deployed to this depth in the open ocean to record data from a specific marine mammal. Whales make distinctive clicking sounds or vocalizations both for communication and for echolocation, allowing them to navigate and forage for food, but traditional acoustic devices on the ocean surface typically can't record whale sounds emitted at lower depths.

The glider is designed to collect acoustic data from beaked whales (Ziphiidae), which can dive down to 2,000 metres. These whales seem to be particularly sensitive to man-made noise, and there have been a number of beaked whale strandings associated with the use of military sonar equipment1.

The data will help to improve our understanding of whale biology, researchers say, but the glider is also being considered as a more effective way of monitoring marine mammals when airguns are deployed for seismic studies of the seafloor (see 'Airgun ban halts seismic tests'). Such tests have been linked to whale strandings or deaths, but when observers try to monitor whales by sight during the studies, "they miss about 85% of the whales present," says whale-acoustics expert Dave Mellinger of Oregon State University's Hatfield Marine Science Center in Newport, Oregon, who works on the glider project.

Since the glider's deployment on 27 October, it has made more than 50 dives — some lasting up to 6 hours — during its first week of operations off the coast of Hawaii. It is due to be retrieved on 17 November.

"We believe we have identified beaked whales," says Mellinger. "It was pretty exciting. You work a couple of years on a project, hope it will succeed, but you don't know until the equipment is wet."

The US$1.5-million project began in 2007 and is funded by the US Office of Naval Research, based in Arlington, Virginia. Mellinger and a postdoctoral researcher, Holger Klinck, also at the Hatfield Marine Science Center, developed the software for the glider so that it could distinguish beaked whale noises from the cacophony of other ocean sounds. At the University of Washington in Seattle, engineers Neil Bogue and Jim Luby conducted the glider engineering research. Mellinger and Klinck's studies are in conjunction with the Pacific Marine Environmental Laboratory in Seattle, Washington, part of the US National Oceanic and Atmospheric Administration.

Sonic search

Developed more than a decade ago, autonomous underwater gliders were initially used by oceanographers to study ocean conditions, such as temperature, salinity and currents. They were dropped off in waters around the world to capture data that were then either recovered with the glider or beamed to a satellite.

A glider can cost around $150,000–200,000 and typically includes a bladder to control its buoyancy. Powered by an internal battery, a device shifts oil, deflating the bladder or filling it with air, and prompting the glider to descend or ascend accordingly.

The two-metre-long, torpedo-like glider used in the beaked whale study is steered by an internal computer to follow a pre-programmed course while travelling at around 0.25 metres per second. After diving, the glider comes to the surface to take its bearings, beams a summary of its data to an Iridium satellite and then cruises to the next programmed location. Cruise distance is dictated by battery life, but as technology has improved, gliders can now travel from the mainland United States to Hawaii.

"The technology — like the acoustic system — wasn't available to do this five years ago," notes Mellinger.

Characteristic click

Two species of beaked whale have been targeted in this particular study — the Cuvier's and the Blainville's. Beaked whales emit around three clicks per second, whereas other marine mammals, such as dolphins, click as often as 100 times per second. Still, recording the beaked whale sounds will result in enormous amounts of information — the current glider cruise is expected to produce a half a terabyte of data.

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To identify the characteristic sound of a beaked whale, Mellinger and Klinck's software must take into account the click's frequency in kilohertz, the rate of clicks and the frequency spectrum of the clicks. The click spectrum emitted by a beaked whale is 25–100 KHz — inaudible to the human ear, which can detect up to a maximum of 20 KHz.

"We have not done detailed analysis, but we are pretty sure we have encountered beaked whales four or five times," says Klinck. Once the glider has finished its cruise, further analysis of the entire data set will confirm and expand that finding. 

  • References

    1. Jepson, P. D. et al. Nature 425, 575-576 (2003). | Article | ISI | ChemPort |
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