The ocean is not a quiet place. Water can move rocks and sediment, even sufficiently to create underwater landslides. Bivalves make clapping noises, fish make sounds during courtship, and cetaceans communicate with clicks and whistles, just to name a few. And of course there is human activity – like shipping, drilling, and sonar, which all add to the sounds of the ocean. There are many different reasons why we might want to hear these noises. Thanks to acoustic monitoring technology we can.
There are many different types of acoustic monitoring equipment but you will tend to find one type of sensor at their heart – the hydrophone. Hydrophones are microphones that can be dropped into the water and listens for sounds coming from any direction. If you have been on a whale-watching boat you may very well have seen one of the crew drop one of these into the water. With the hydrophone, the crew can hear a noisy whale and even work out their location. In some places, hydrophones are anchored to the sea floor and float in the water column recording any sounds within their range, until their battery runs out and/or they are picked up again by boat.
Hydrophones can be cabled together, and attached to an underwater communications cable too. Perhaps the most famous example is the US Navy’s SOSUS system which has been in operation since the 1960s. The devices were originally intended for military use, but since the late 1980s the scientific community have been able to listen in as well. By grouping the hydrophones together, you create an array which has greater sensitivity than a single hydrophone. There is another advantage these cabled systems have too. Because they are attached to a communication cable, researchers can obtain data in real-time. More recently, hydrophones have been attached to gliders – an autonomous underwater vehicle (but more on that in a later post), allowing hydrophones to follow mobile species like sei whales over large distances.
So what has all this eavesdropping taught us about the life aquatic? Here are just a few open access pieces of research that have used acoustic monitoring:
Singing with your dinner
Whales are famous for their song. Male humpback whales (Megaptera novaeangliae) sing extensively during their breeding season, most probably to attract the ladies (who doesn’t love a good singing voice!). The thing is, the whales have also been caught singing outside their breeding grounds, out of season. Between 2006 and 2008 Elizabeth Vu of NEFSC, and colleagues from elsewhere in the USA implemented a long-term acoustic survey in the Stellwagen Bank National Marine Sanctuary which lies in the Northwest Atlantic and forms an important feeding ground for the humpbacks. They detected extensive whale song starting from the moment the whales arrived at the feeding ground. As the months went on and the time to migrate back down south to their breeding grounds came closer, the singing males opted not only to sing for longer but to sing more often. Interestingly, singing males were detected in the feeding ground well into the breeding season. Apparently not all males want to migrate every year. http://ow.ly/M8zKy
Stalking a blue whale
Finding whales – even ones as large as blue whales – is no easy task. When the Antarctic Blue Whale project went out and did surveys for the Antarctic blue whale (Balaenoptera musculus intermedia) in the Southern they didn’t find many. So few in fact, that it was time to rethink their strategy. Whales make sounds and sound travels in the ocean…. Enter acoustic monitoring. In 2012 Brian Miller from the Australian Marine Mammal Centre explored the idea of using FIFAR sonobuoys – basically 3 hydrophones attached to a buoy with a compass, some signal processing and recording gear, and a VHF radio to track the pygmy blue whale (Balaenoptera musculus brevicauda) . The VHF radio isn’t exactly high-tech gear but it was vital because the radio broadcast the sonobuoy data to a boat-based group of visual observers. In hearing the VHF, the team would then try and locate the whales. And it seemed to work. Out of the 32 individual whales identified, 90% (29) were also spotted by the visual survey team. http://ow.ly/M8zOj
Can you hear me
With all that human activity the oceans have gotten a lot noisier. There are an increasing number of studies looking at how different species are responding to the noise – including Dugongs in Thailand. In this study Noriko Ando-Mizobata from Kyoto University and his colleagues from Japan used an Automatic Underwater SOund Monitoring System for Dugong (AUSOMS-D) to monitor how dugongs on the south coast of Talibong Island were altering the calls during noisy and less noisy times. Ambient noise was highest during the day and quietest in the very early hours of the morning, seeming to coincide with boat traffic. When it was noisiest, the dugongs appeared to increase their call frequency to ‘shout’ above the noise (or moved closer to the AUSOMS-D during noisy periods…). So on one side it’s good they can still communicate, but on the other increasing call volume uses more energy, and if boat traffic increases above the capabilities of the Dugongs vocalisations…well they are in trouble. http://ow.ly/M8zQD
Check out one of my older posts
I wrote a post last year on Google + that covers an open access paper which used acoustic monitoring to uncover three distinct acoustic populations of pygmy blue whale in the Indian Ocean. Very cool! Check it out here http://t.co/65KQ2XNEpb
Image: Dolphins at Key West, Florida. Credit Jay Ebberly/Flickr (CC BY 2.0)