Effects of Airgun Sounds on Bowhead Whale Calling Rates: Evidence for Two Behavioral Thresholds

In proximity to seismic operations, bowhead whales ( Balaena mysticetus ) decrease their calling rates. Here, we investigate the transition from normal calling behavior to decreased calling and identify two threshold levels of received sound from airgun pulses at which calling behavior changes. Data were collected in August–October 2007–2010, during the westward autumn migration in the Alaskan Beaufort Sea. Up to 40 directional acoustic recorders (DASARs) were deployed at five sites offshore of the Alaskan North Slope. Using triangulation, whale calls localized within 2 km of each DASAR were identified and tallied every 10 minutes each season, so that the detected call rate could be interpreted as the actual call production rate. Moreover, airgun pulses were identified on each DASAR, analyzed, and a cumulative sound exposure level was computed for each 10-min period each season ( CSEL 10-min ). A Poisson regression model was used to examine the relationship between the received CSEL 10-min from airguns and the number of detected bowhead calls. Calling rates increased as soon as airgun pulses were detectable, compared to calling rates in the absence of airgun pulses. After the initial increase, calling rates leveled off at a received CSEL 10-min of ~94 dB re 1 μPa 2 -s (the lower threshold). In contrast, once CSEL 10-min exceeded ~127 dB re 1 μPa 2 -s (the upper threshold), whale calling rates began decreasing, and when CSEL 10-min values were above ~160 dB re 1 μPa 2 -s, the whales were virtually silent.