Sounds in the Ocean

Antarctica +++ 70° 33’ S, 8° 50’ W +++ --‐3°C, cloudy, Northerly winds +++
08 January 2015
(by Karolin Thomisch, Stefanie Spiesecke and Ina Lefering, translation by Ina Lefering)

We, the ‘Ocean Acoustics’ group, study the distribution of marine mammals (such as whales, dolphins, and seals) in the southern ocean. In order to study the animals and the waters around, we include two types of acoustic devices into the moorings that are deployed during this expedition: acoustic underwater recorders and sound sources. The recorders are equipped with a hydrophone, which ‘listens’ into the water column and records continuously under water noise.

Most marine mammals make species specific sounds and it is possible to distinguish different species by their vocalisation. You can tell from the acoustic recordings when which species is in the area around a recorder. We study the distribution of species in time and space and, for example, can identify if a species is present in the Weddell Sea throughout the entire year or if it leaves the area during a certain period of the year. Humpback whales, for example, are known for complex vocalisations. With our instruments, we were able to show that they stay close to the ice shelf for the entire year – even in winter, when the ocean is almost completely covered with sea ice.

Besides the recorders in moorings that are deployed throughout the Weddell Sea, we also run the acoustic observatory PALAOA located on the Ekstrøm ice shelf. PALAOA has been recording continuously everything that ‘happens’ close to the ice edge since 2005: varying from ice berg calving to the calls of Ross Seals with their characteristic and futuristic sound.The second type of instruments, our sound sources, is a key element for the floats (autonomous profilers) program in sea ice regions. Floats drift autonomously through the oceans and move up and down the water column to collect profiles on sea water properties (e.g. temperature and salinity).

Data and position are transferred via satellite once the float reaches the sea surface. Our floats are equipped with an additional hydrophone to receive the acoustic signals of the sound sources. If a float is unable to reach the sea surface due to a dense sea ice cover, we can determine its position afterwards using the frequently received acoustic signals. Each sound source emits an acoustic signal with a frequency of about 260 Hz every day at the same time. Through measuring how long the signal of a certain sound source has travelled from the mooring to the float, we can determine the position of the float within a few kilometres. Therefore, it needs to receive signals from at least 3 different sound sources.

Impressions