Listening to the blues
Antarctic scientists have tracked and located more than 50 blue whales using acoustic technology to eavesdrop on the animals’ resonant song.
By using sound rather than sight to initially detect the whales, the scientists greatly improved the likelihood of finding and counting whales in the vast Southern Ocean, saving enormous amounts of searching effort and expensive ship time.
To test the technology the team, led by Australian Antarctic Division marine biologist Dr Mike Double, deployed 131 ‘directional sonobuoys’ in northern Bass Strait in January and March 2012.
‘Previous methods of estimating blue whale abundance by sightings surveys from ships, allowed us to visually detect a whale from up to 10 km away in good weather, but acoustic methods can allow you to detect them perhaps as much as several hundred kilometres away, and in rough weather,’ Dr Double says.
‘We wanted to answer questions such as how far away can we detect whales using the sonobuoys, how far away can the vessel pick up the VHF signal from the sonobuoys, do whales sing long enough for us to find them, and can we track the whales at night?’
In the first of the two three-week voyages the sonobuoys were deployed as needed in a 500 km study area south of Portland, Victoria. Australian Antarctic Division acoustician, Dr Brian Miller, said the first 10 sonobuoys detected nothing but ship and wave noise, as they traversed an area apparently devoid of blue whales. But it wasn’t long after the ship moved to another region that the team struck gold.
‘We heard our first blue whale singing at 2 am and tracked the ship towards the sound through to 6 am, when the whale stopped singing,’ Dr Miller says.
The team estimated that the ship had approached the singing whale to within about two kilometres. At day-break the observer team then visually detected the animal and directed the ship to approach the whale during several surfacing bouts, before it surfaced some 50 m from the vessel, allowing them to take photographs for individual identification.
The team continued this process of deploying sonobuoys to triangulate the location of singing whales and get within range to make visual sightings, with good success.
‘With a team of five dedicated acousticians we are able to monitor for whales 24 hours a day, every day. This continuous acoustic coverage ensured that the ship was almost always moving towards whales, so as to maximise the number of encounters,’ Dr Miller says.
‘During both voyages, 32 vocalising blue whales were pursued using acoustic tracking and of these we sighted 29 groups of one or more whales – a 90 per cent success rate.
‘We monitored more than 500 hours of audio in real time, yielding over 20 000 blue whales calls, and our tracking enabled 70 blue whale sightings on the first voyage and 34 on the second.’
Dr Miller says that over the continental shelf, distances to acoustically tracked whales were typically less than 20 km. However, in the deeper waters further offshore, the team successfully tracked whales over distances greater than 60 km.
‘These long-distance, deep-water tracks are very important as they are most similar to the conditions we expect to encounter when tracking blue whales in the Southern Ocean around Antarctica,' Dr Miller says.
The team also trialled a prototype of a moored acoustic recorder, which remained anchored to the sea floor (at about 800 m depth) for three days. The mooring, developed and built at the Australian Antarctic Division, successfully recorded whale song during this time and, in the future, could be used for longer-term deployments of up to 15 months.
The acoustic technology and methodology will now be used on blue whales in Antarctic waters in February 2013, during the inaugural voyage of the Antarctic Blue Whale Project. This flagship project of the Southern Ocean Research Partnership aims to estimate the abundance of blue whales in the Southern Ocean, 50 years after whalers killed some 350 000 individuals, as well as examine their distribution, population structure and migration routes.
‘This pilot study has given us confidence that the sonobuoys will help us to find rare blue whales in Antarctica,’ Dr Double says.
Australian Antarctic Division