Abstract:

An NOAA/ONR acoustic line-transect survey was conducted in spring 1997 to estimate the abundance of sperm whales in the eastern temperate North Pacific using a 52-m research vessel. An ITI hydrophone array was towed at 100-m depth as the vessel surveyed approximately 25 000 km of predetermined transect lines at 8 kn. Acoustic signals received by a pair of hydrophones were digitized and sent up to the ship via a coaxial tow cable. Bearing angles to the locations of sperm whales were calculated in real time based on differences in signal arrival times between the two hydrophones, and whale locations were determined by the convergence of bearing lines as the ship continued along its course. The ship was maneuvered to resolve the left/right ambiguity in whale locations and, during daylight hours, was directed to this estimated location to obtain visual estimates of group size when the whales surfaced. A total of 102 distinct groups of sperm whales were acoustically detected with sufficient amplitude to estimate bearing lines. ``Slow clicks'' (with a period of 4--8 s) were detected at ranges up to 20 nmi, whereas the more typical sperm whale clicks (with a period of 0.5--1.0 s) were typically heard at less than 5 nmi.