Underwater Tracking of Humpback Whales (Megaptera Novaeangliae) With High-Frequency Pingers and Acoustic Recording Tags

A long-baseline (LBL) acoustic system has been developed for the tracking of humpback whales (Megaptera novaeangliae) that have been tagged with digital acoustic recording devices (DTAGs), providing quantitative observations of submerged whale behavior during bubble net feeding. The system includes three high-frequency acoustic sources deployed from small boats that follow the whale after the animal has been tagged. Integrated global positioning systems (GPSs) provide positioning and synchronized operation of the sources. Time-encoded acoustic signals from the sources are recorded along with whale vocalizations and ambient noise on the whale tag. Time-of-flight measurements, as measured by the tag acoustic data, are converted to range from the whale to each source with a measured sound-speed profile. A nonlinear least squares solution is then solved for the whale's position with a nominal positional fix rate of once per second. The system is demonstrated with data collected from a tagged animal in summer 2007. Dead-reckoned track generation methods commonly used in previous studies are shown to capture the qualitative nature of the whale track, albeit with poor absolute positional accuracy, and to distort the track when the whale's movement is predominantly vertical. In contrast, the LBL data can provide quantitative measures of whale behavior. Transit speeds between bubble net feeding events for this case study are found to range from 0.7 to 1.9 m . s(-1) (n = 8). The mean diameter of bubble net curtains are measured to range from 9.6 to 10.9 m. Whale speeds during bubble net rotations vary from 1.0 to 1.9 m . s(-1) (n = 6).