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Scientist find source of mysterious Southern Ocean sound

New data could help minke whale conservation efforts

A group of Antarctic minke whales. Photo courtesy Ari S. Friedlaender, Oregon State University

A group of Antarctic minke whales, which have been identified as the source of a mysterious sound in the Southern Ocean. Photo courtesy Ari S. Friedlaender, Oregon State University.

Staff Report

FRISCO — If you’ve ever heard mysterious sounds that you can’t identify, you’re not alone. For decades, researchers have tried to trace the source of a unique rhythmic sound in the remote Southern Ocean that’s often been recorded, but never definitively pinpointed — until now.

This week, scientists with NOAA’s Northeast Fisheries Science Center said the sound is generated by the Antarctic Minke whale, the smallest of the “great whales” or rorquals, a group that includes the blue whale, Bryde’s whale, and humpback, fin, and sei whales. Rorqual whales are relatively streamlined in appearance, have pointed heads and, with the exception of humpback whales, small pointed fins.

The sound, described as a series of pulses in a highly repetitive pattern, was first described and named by submarine personnel in the 1960s who thought it sounded like a duck. Early speculation as to the source of the sound focused on submarines, oceanographic phenomena, or even fish.

The identification of the Antarctic minke whale as the source of the sound now indicates that some minke whales stay in ice-covered Antarctic waters year-round, while others undertake seasonal migrations to lower latitudes. The bio-duck sound is heard mainly during the austral winter in the Southern Ocean around Antarctica and off Australia’s west coast.

“These results have important implications for our understanding of this species,” said Risch, a member of the Passive Acoustics Group at the NEFSC’s Woods Hole Laboratory. “We don’t know very much about this species, but now, using passive acoustic monitoring, we have an opportunity to change that, especially in remote areas of the Antarctic and Southern Ocean.”

The researchers were able to trace the sound by deploying acoustic tags on two Antarctic minke whales in Wilhelmina Bay off the western Antarctic Peninsula. These tags were the first acoustic tags successfully deployed on this species. The acoustic analysis of the data, which contained the bio-duck sound, was led by Denise Risch of NOAA’s Northeast Fisheries Science Center (NEFSC) and was published April 23, 2014 in Biology Letters.

The acoustic tags, which also recorded water temperature and pressure, were placed on the animals using a hand-held carbon fiber pole by researchers working from a rigid-hulled inflatable boat. Animals were visually tracked from the boat during daylight hours to identify behavior and group composition. No other marine mammal species were observed in the area when calls were recorded, providing further evidence that the recorded sounds were produced by the tagged whale or other nearby Antarctic minke whales.

The authors said that identifying the bio-duck sound will allow for broader studies of the presence of minke whales in other seasons and areas. That ability to monitor minke whales is critical for a species that inhabits an environment that is difficult to access, has rapidly changing sea-ice conditions, and “has been the subject of contentious lethal sampling efforts and international legal actions.”

In addition to lead author Risch and NEFSC colleague Sofie van Parijs, other authors represent the Australian Antarctic Division in Tasmania, Australia; NOAA Fisheries Office of Science and Technology in Silver Spring, Maryland; Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany;  Duke University Marine Laboratory in Beaufort, North Carolina; Institute for Terrestrial and Aquatic Wildlife Research, University of Veterinary Medicine Hannover Foundation in Büsum, Germany; and the Marine Mammal Institute, Hatfield Marine Science Center at Oregon State University in Newport, Oregon.

The study was supported by a grant from the National Science Foundation’s Office of Polar Programs and conducted under National Marine Fisheries Service Permit 14097, Antarctic Conservation Act Permit 2009-013, and Duke University Permit IACUCA49-12-02. Denise Risch’s research was also supported by the U.S. Navy Environmental Readiness Division (N45).

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