Findings on background sounds in Southern Ocean published
For nearly three years, using underwater recording devices, scientists have been listening in to a chorus of sounds made by whales and seals in the Southern Ocean. These sounds have providing new insight into the natural sounds of the ocean as well as the behaviour and distribution of these animals.
It is never completely silent in the ocean. Wind and waves create a continuous sound in the background. There are also sounds from human activities like shipping or seismic exploration that can sometimes thunder over the natural sounds. However, the Antarctic remained largely unaffected due to its isolated location. Because of this, the Southern Ocean is an ideal location to conduct an acoustic study on marine mammals and underwater background sounds. For nearly three years, scientists from the working group on oceanic acoustics at the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI) have conducted this study in the Antarctic. Their results are now published in the Royal Society Open Science journal.
Lead author Sebastian Menze and colleagues have identified, among other things, the sounds of leopard seals, Antarctic blue whales, fin whales and Southern minke whales, within the chorus of monotonous background sounds. The animal sounds vary in their timing and pace, thereby providing fresh insight into the animals' behaviour and distribution.
The sounds of the Southern dwarf whale, for example, follow a 24-hour rhythm during the winter months of April to July: The recording showed that the dwarf whales are more vocal during the day. This may be because their main prey, krill, migrate vertically in an identical day-night rhythm. In addition, the scientists also collected data on the cycle of the animals during the season. For example, Antarctic blue whales contributed to the sounds all year round, while fin whales and Southern minke whales did so for only a few months.
The marine biologists and physicists also discovered how sea ice influenced the amount of sounds in the Southern Ocean. In the winter months, it is laid over the ocean like a soundproof carpet: “In Antarctica, it is remarkably quiet under the ice cover. The sounds no longer comprise those of physical phenomena like storms or waves, but originate from the animal world,” said Menze in German. The acoustic recordings show that not only the surface of the sea ice plays a role, but also its concentration and character.
For the study, the scientists moored two autonomous underwater acoustic recorders in the Atlantic sector of the Southern Ocean from March 2008 to December 2010, at 217 and 260 metres depth. It is the first long-term study on underwater sounds conducted in the higher latitudes of the Antarctic Ocean.