Passive Acoustic Monitoring Arrays

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Seafloor and moored hydrophones that record low-frequency whale calls continuously, over ranges that a ship or an aircraft cannot cover. Blue whale calls are very low in frequency and propagate efficiently in the deep sound channel, so a single recorder covers a large area. The technique was inherited rather than invented for whales: the US Navy SOSUS arrays, built from the mid-1950s to track submarines, were declassified for their mission in 1991, and the distinctive 52 Hz call had already been identified from Navy hydrophone data in 1989 by William Watkins at Woods Hole. The Antarctic application is now large - the IWC-SORP acoustic trends group had access to more than 300,000 hours of Southern Ocean passive acoustic recordings as of March 2020, principally listening for the three-part Antarctic blue whale Z-call that sweeps from about 28 Hz to about 17 Hz over as much as 18 seconds.