Directionality of bowhead whale calls measured with multiple sensors

Citation

Blackwell, S. B., T. L. McDonald, K. H. Kim, L. A. M. Aerts, W. J. Richardson, C. R. Greene, and B. Streever (2012). “Directionality of bowhead whale calls measured with multiple sensors”. In: Marine Mammal Science 28.1, pp. 200-212. DOI: 10.1111/j.1748-7692.2011.00464.x.

Number of Citations: 17

Abstract

Directionality (directivity) in animal vocalizations—the tendency for a call to radiate more strongly in some directions than others—is well documented in echolocating bats and odontocetes but poorly known among baleen whales. We tested whether the calls of bowhead whales (Balaena mysticetus) are directional in the horizontal plane during their autumn migration through the Alaskan Beaufort Sea. An array of Directional Autonomous Seafloor Acoustic Recorders (DASARs) was deployed offshore of the Northstar oil-production island near Prudhoe Bay from 27 August to 25 September 2008. Assuming migrating whales were oriented roughly west-northwest, we compared received levels (RLs) of calls at recorders positioned “ahead” versus “behind” the whales. To separate true directionality from environmental propagation effects, each of 46 whale calls was matched to the nearest omnidirectional calibration transmission and the difference of differences (\(\Delta\)RL) was analyzed. Bowhead calls were significantly stronger toward the front of the whale, averaging 4.8 dB (E–W recorder pairs) and 4.2 dB (SSE–NNW pairs) higher ahead than behind; after accounting for the calibration signals, forward directionality averaged 3.9 and 3.3 dB (P \(\le\) 0.0004). Complex calls and upcalls showed forward directionality, whereas constant calls did not. The effect may arise from reflection and absorption of sound by the whale’s air-filled lungs, which taper from front to back. Because directionality biases which calls seafloor recorders detect, it can distort apparent call distributions and should be accounted for in studies that monitor whales acoustically.