Validating the reliability of passive acoustic localisation: a novel method for encountering rare and remote Antarctic blue whales

Since its near extirpation during the period of industrial whaling in the early and mid 20th century, the once common Antarctic blue whale Balaenoptera musculus intermedia re mains extremely rare. While annual systematic surveys around Antarctica from 1978 to 2009 re corded only 216 visual encounters of this species, their loud and distinctive calls were detected frequently throughout the Southern Ocean. We describe and assess a new method for locating these whales by acoustically detecting their vocalisations, tracking the location of their calls, and finally locating the whales visually. This methodology was employed during an Antarctic research voyage from 140° E to 170° W, between January and March 2013. The loudest song unit (a 26 Hz tone) was detected at all 298 recording sites south of 52° S. Acoustically derived bearings from these whales enabled visual observers to eventually sight the whales, often hundreds of kilometres from initial acoustic detections. Received sound pressure levels of detections increased with decreasing range to several hotspots where both song and non-song calls were detected. Within hotspots, short- range acoustic localisation yielded 33 visual encounters of Antarctic blue whales (group size: 1 to 5 whales) over a 31 d period south of 60° S. These results demonstrate that acoustic tracking pro- vides the capacity to locate Antarctic blue whales widely dispersed over many hundreds of kilo- metres, as well as the capacity to acoustically track individual whales for days at a time irrespec- tive of most weather conditions. Thus, passive acoustic localisation is a reliable and efficient method to track Antarctic blue whales, and this technique should be considered for future studies of these iconic animals.