Long-range acoustic tracking of Antarctic blue whales

The Southern Ocean Research Partnership (SORP) has developed an Antarctic Blue Whale Project which includes research to derive a mark-recapture abundance estimate for Antarctic blue whales. Tracking blue whales through passive acoustic monitoring has been identified as a potential means for increasing encounter rates, and thus facilitating abundance estimates through photo-identification and biopsy. This methodology was pursued by the Australian Antarctic Division (AAD) using DIFAR sonobuoys to detect, localise and track Antarctic blue whales on a research cruise from 140 E to 165 W, and south of 60S between January and March 2013. Antarctic blue whales make loud and distinctive calls, known as ‘Z’ and ‘D’ calls. The loudest element of the ‘Z’ call (a 26Hz tone) was detected at a range of hundreds of kilometres. 26Hz calls were detected on all DIFAR sonobuoys deployed south of 52S (n= 298). Whilst overlapping calls sometimes merged into a continuous tone, it was still possible to select individual calls which could be localised. Bearings from these vocalising whales allowed them to be acoustically tracked and targeted. Multiple sonobuoys were used to triangulate the location of individuals and groups. Received levels of detections increased with decreasing range to several acoustic ‘hotspots’ in the survey area, where whales were sighted. At these closer distances, full ‘Z’ calls and ‘D’ calls were also detected. 85% of acoustic targets resulted in visual encounters, yielding 32 encounters with groups of blue whales. The results demonstrate the ability of acoustic tracking to locate Antarctic blue whales that are widely dispersed over a large area as well as the capacity to acoustically track whales for days at a time. These abilities may assist with characterising their behaviour in their Antarctic feeding grounds. The results from this study may serve as a benchmark for future acoustic surveys of Antarctic blue whales, and may also be useful for quantifying the effects of acoustic tracking when designing future surveys.