Killer Whale (Orcinus orca) Acoustic Responses to Naval Sonar

Regulators and navies need to estimate the effects of underwater noise, such as naval sonar, on cetaceans. In this study, we investigated acoustic responses of four killer whale (Orcinus orca) groups exposed to naval sonar in experiments that contained one or three exposure sessions. Using DTAGs, we compared group acoustic behaviour before, during, and after exposure sessions. Groups feeding and acoustically active prior to exposure (n = 2) significantly changed call rate and call overlap, and increased the maximum frequency of their calls during 1 to 2 kHz or 6 to 7 kHz sonar exposures. One group that switched from feeding to travelling prior to exposure also changed its call rate. In one session, there was a noticeable increase in call rate timed closely to sonar transmissions, with high call overlap and the production of highly frequency-modulated calls. During the remaining two sessions in which whales were calling but not feeding prior to exposure, there were no significant changes in calling behaviour, although high-frequency whistles were recorded in one case when a calf briefly separated from its group. The results from these case studies indicate that vocal responses, concurrent with changes from feeding to travel, reflect changes in the animals’ behavioural state, though, as of yet, undetermined effects of behavioural context likely influence acoustic responses to sonar. Further data will be needed to advance our understanding of the drivers behind vocal adjustments in different noise and behavioural contexts.