Listening to ocean sounds: using underwater acoustics to study marine animals

Marine animals make extensive use of sound in an environment where vision is usually very limited but where sound travels to great distances. Their sounds can provide information about their distributions and abundance as well as providing insights into their behaviour. This requires more than just studying their vocalisations – it also requires knowledge of their acoustic environment and the way that this affects their use of sound. This paper presents some results of recent experiments illustrating these themes, including some measurements of the acoustic environment required to interpret the behavioural studies. Beaked whales are small elusive whales that are rarely seen and little is known of their distributions. Visual surveys are usually ineffective. Cruises in the Coral Sea in 2008 and 2009 trialled acoustic methods of detecting beaked whales and surveyed an area of more than 19,000 km2 which included continental shelf, slop and deep water. Large numbers of their distinctive click sounds were detected in deep water near the continental slope and the steep sides of islands. Humpback whales are very vocal and their use of sound is an important part of their behaviour. In a series of experiments, we have studied the behaviour and sound production of migrating humpback whales and tracked their movements using their sounds. As part of this we measured the transmission loss of the beach side study area to allow us to calculate the source levels (vocal outputs) of the whales from the received levels, and how these varied with behaviour and background noise. White noise of known source level, filtered in octave bands, was transmitted using an underwater loudspeaker. Measurements at various distances showed that the transmission loss for the sloping beach area was a little less than expected for free field transmission (spherical spreading).