Scientists identify source of mysterious sound in the Southern Ocean

In February 2013, an international team of researchers deployed 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 bio-duck sound is heard mainly during the austral winter in the Southern Ocean around Antarctica and off Australia's west coast. Described as a series of pulses in a highly repetitive pattern, the bio-duck's presence in higher and lower latitudes during the winter season also contributed to its mystery. No one knew the minke whales were there. 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.

"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 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 mysterious sounds were thought to be made by submarines, by some oceanographic phenomenon, or even by fish. They were eventually identified as the bio-duck through comparisons with sounds in the published literature. They also matched recordings on long-term, bottom-mounted recorders from several other locations in the Antarctic, including the Perennial Acoustic Observatory in the Antarctic Ocean (PALAOA), and near Dumont D'Urville and Ross Island. Germany's PALAOA is an autonomous observatory with underwater hydrophones, or microphones, located on the Ekström Ice Shelf in western Antarctica. Dumont d'Urville on Petrel Island is the main French scientific research station in Eastern Antarctica.

Findings from this study will allow researchers to interpret numerous long-term, acoustic recordings, and improve understanding of the distribution, abundance, and behavior of this species. Minke whales are 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 authors note 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."

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).