Real-time tracking of blue whales using DIFAR sonobuoys

Passive acoustic tracking of blue whales has been proposed as a key component in a strategy to obtain a circumpolar abundance for Antarctic blue whale as proposed in a project of the IWC Southern Ocean Research Partnership. While a theoretical basis for passive tracking of blue whales has been demonstrated, there are substantial differences between these theoretical scenarios and a fully-operational, dedicated, real-time, tracking program. Because passive acoustic tracking relies upon the use of complex electronic systems, it is important to understand the capabilities and limitations of the hardware and software that comprise the system in order to understand what constitutes sensible use. We present preliminary results from two research voyages where a fully-operational, dedicated, real-time tracking system was used to locate blue whales over distances of more than 60 km. In order to test the tracking system locally before embarking upon an expensive Antarctic voyage, pygmy blue whales in Northern Bass Strait were used as a surrogate for Antarctic blue whales in the Southern Ocean. The core element of the acoustic tracking system consisted of DIFAR sonobuoys, VHF radio receivers, and custom analysis software driven round the clock by a team of acousticians. The tracking system operated continuously during the voyages, recording nearly 500 hours of audio, while acousticians processed over 7000 blue whale calls all in “real-time”. During the 20 days at sea 32 vocalising blue whales were “targeted” and, of these, 29 yielded visual sightings of one or more blue whales giving a combined success rate greater than 90%. While there are many differences between the Bass Strait and the colder waters around Antarctica, acoustic detection ranges of blue whales in the Southern Ocean far outstrip visual sighting ranges, so real-time acoustic tracking may be able to increase the total number of whale encounters, thus making more efficient use of expensive ship time. The success of the real-time tracking system during these two voyages supports the use of acoustics as a key tool in determining the circumpolar abundance of blue whales in the Southern Ocean.