Calibration and comparison of the acoustic location methods used during the spring migration of the bowhead whale, Balaena mysticetus, off Pt. Barrow, Alaska, 1984-1993

Between 1984 and 1993, visual and acoustic methods were combined to census the Bering-Chukchi-Beaufort bowhead whale, Balaena mysticetus, population. Passive acoustic location was based on arrival-time differences of transien t bowhead sounds detected on sparse arrays of three to five hydrophones dis tributed over distances of 1.5-4.5 km along the ice edge. Arrival-time diff erences were calculated from either digital cross correlation of spectrogra ms (old method), or digital cross correlation of time waveforms (new method ). Acoustic calibration was conducted in situ in 1985 at five sites with vi sual site position determined by triangulation using two theodolites. The d iscrepancy between visual and acoustic locations was <1%-5% of visual range and less than 0.7 degrees of visual bearing for either method. Comparison of calibration results indicates that the new method yielded slightly more precise and accurate positions than the old method. Comparison of 217 bowhe ad whale call locations from both acoustic methods showed that the new meth od was more precise, with location errors 3-4 times smaller than the old me thod. Overall, low-frequency bowhead transients were reliably located out t o ranges of 3-4 times array size. At these ranges in shallow water, signal propagation appears to be dominated by the fundamental mode and is not corr upted by multipath. (C) 2000 Acoustical Society of America. [S0001-4966(00) 01006-7].