Acoustic response of a migrating bowhead whale population to open-ocean ambient noise levels

Automated and manual acoustic localizations of migrating bowhead whale calls in the Beaufort Sea were used to examine their acoustic response to changes in wind-driven, continuous, ambient noise levels. At low noise levels, the population’s source levels and calling rates increased with increasing noise level, with the source level distribution adjusting to maintain a consistent functional detection range, estimated to be between 20 and 60 km. However, once noise levels exceeded the 75thpercentile of their long-term distribution, source level increases failed to keep pace with further increases in noise level, thereby reducing the population’s detection range and associated communication space. Call production rates, on the other hand, continued to increase even up to the highest noise levels. Migrating bowhead whales thus attempted to maintain long-range call detectability by adjusting their source level and calling rate. Beyond a certain noise level, whales cannot increase their source levels, but do continue to increase their calling rate. The results provide context for interpreting the effects of industrial noise on bowhead whale acoustic behavior; for example, distant airgun signals stimulate an increase in mean call production rate equivalent to a 26 dB increase in natural ambient noise levels. [Work sponsored by NPRB.]