This study describes behavioral changes of wild cetaceans observed during controlled exposures of naval sonar. In 2006 through 2009, 14 experiments were conducted with killer (n = 4), longfinned pilot (n = 6), and sperm (n = 4) whales. A total of 14 6-7 kHz upsweep, 13 1-2 kHz upsweep, and five 1-2 kHz downsweep sonar exposures, as well as seven Silent vessel control exposure sessions and eight playbacks of killer whale sounds were conducted. Sonar signals were transmitted by a towable source that approached each tagged subject from a starting distance of 6 to 8 km with a ramp up of source levels (from 152 to 158 to a maximum of 198 to 214 dB re: 1 µPa m). This procedure resulted in a gradual escalation of the sonar received level at the whale, measured by towed hydrophones and by tags that record movement and sound (Dtags). Observers tracked the position of each tagged animal and recorded group-level surface behavior. Two expert panels independently scored the severity of diverse behavioral changes observed during each sonar and control exposure, using the 0 to 9 point severity scale of Southall et al. (2007), and then reached consensus with a third-party moderator. The most severe responses scored (i.e., most likely to affect vital rates) included a temporary separation of a calf from its group, cessation of feeding or resting, and avoidance movements that continued after the sonar stopped transmitting. Higher severity scores were more common during sonar exposure than during Silent control sessions. Scored responses started at lower sound pressure levels (SPLs) for killer whales and were more severe during sonar exposures to killer and sperm whales than to longfinned pilot whales. Exposure sessions with the higher source level of 1 to 2 kHz sonar had more changes and a trend for higher maximum severity than 6 to 7 kHz sessions, but the order of the sessions had no effect. This approach is helpful to standardize the description of behavioral changes that occurred during our experiments and to identify and describe the severity of potential responses of free-ranging cetaceans to sonar.

Eight experimentally controlled exposures to 1À2 kHz or 6À7 kHz sonar signals were conducted with four killer whale groups. The source level and proximity of the source were increased during each exposure in order to reveal response thresholds. Detailed inspection of movements during each exposure session revealed sustained changes in speed and travel direction judged to be avoidance responses during six of eight sessions. Following methods developed for Phase-I clinical trials in human medicine, response thresholds ranging from 94 to 164 dB re 1 lPa received sound pressure level (SPL) were fitted to Bayesian dose-response functions. Thresholds did not consistently differ by sonar frequency or whether a group had previously been exposed, with a mean SPL response threshold of 142 6 15 dB (mean 6 s.d.). High levels of between-and within-individual variability were identified, indicating that thresholds depended upon other undefined contextual variables. The dose-response functions indicate that some killer whales started to avoid sonar at received SPL below thresholds assumed by the U.S. Navy. The predicted extent of habitat over which avoidance reactions occur depends upon whether whales responded to proximity or received SPL of the sonar or both, but was large enough to raise concerns about biological consequences to the whales.

Marine mammals may be negatively affected by anthropogenic noise. Behavioural response studies (BRS) aim to establish a relationship between noise exposure conditions (dose) from a potential stressor and associated behavioural responses of animals. A recent series of BRS have focused on the effects of naval sonar sounds on cetaceans. Here, we review the current state of understanding of naval sonar impact on marine mammals and highlight knowledge gaps and future research priorities. Many marine mammal species exhibit responses to naval sonar sounds. However, responses vary between and within individuals and populations, highlighting the importance of exposure context in modulating dose–response relationships. There is increasing support from both terrestrial and marine systems for the risk‐disturbance hypothesis as an explanation for underlying response processes. This proposes that sonar sounds may be perceived by animals as a threat, evoking a response shaped by the underlying species‐specific risk of predation and anti‐predator strategy. An understanding of responses within both the dose–response and risk‐disturbance frameworks may enhance our ability to predict responsiveness for unstudied species and populations. Many observed behavioural responses are energetically costly, but the way that these responses may lead to long‐term individual and population‐level impacts is poorly understood. Synthesis and applications. Behavioural response studies have greatly improved our understanding of the potential effects of naval sonar on marine mammals. Despite data gaps, we believe a dose‐response approach within a risk‐disturbance framework will enhance our ability to predict responsiveness for unstudied species and populations. We advocate for (1) regulatory frameworks to utilize peer‐reviewed research findings when making predictions of impact, (2) regulatory frameworks to account for the inherent uncertainty in predictions of impact and (3) investment in monitoring programmes that are both directed by recent research and offer opportunities for validation of predictions at the individual and population level.

Decompression sickness (DCS; ‘the bends’) is a disease associated with gas uptake at pressure. The basic pathology and cause are relatively well known to human divers. Breath-hold diving marine mammals were thought to be relatively immune to DCS owing to multiple anatomical, physiological and behavioural adaptations that reduce nitrogen gas (N2) loading during dives. However, recent observations have shown that gas bubbles may form and tissue injury may occur in marine mammals under certain circumstances. Gas kinetic models based on measured time-depth profiles further suggest the potential occurrence of high blood and tissue N2 tensions. We review evidence for gas-bubble incidence in marine mammal tissues and discuss the theory behind gas loading and bubble formation. We suggest that diving mammals vary their physiological responses according to multiple stressors, and that the perspective on marine mammal diving physiology should change from simply minimizing N2 loading to management of the N2 load. This suggests several avenues for further study, ranging from the effects of gas bubbles at molecular, cellular and organ function levels, to comparative studies relating the presence/absence of gas bubbles to diving behaviour. Technological advances in imaging and remote instrumentation are likely to advance this field in coming years.

Although northern bottlenose whales were the most heavily hunted beaked whale, we have little information about this species in its remote habitat of the North Atlantic Ocean. Underwater anthropogenic noise and disruption of their natural habitat may be major threats, given the sensitivity of other beaked whales to such noise disturbance. We attached dataloggers to 13 northern bottlenose whales and compared their natural sounds and movements to those of one individual exposed to escalating levels of 1–2 kHz upsweep naval sonar signals. At a received sound pressure level (SPL) of 98 dB re 1 μPa, the whale turned to approach the sound source, but at a received SPL of 107 dB re 1 μPa, the whale began moving in an unusually straight course and then made a near 180° turn away from the source, and performed the longest and deepest dive (94 min, 2339 m) recorded for this species. Animal movement parameters differed significantly from baseline for more than 7 h until the tag fell off 33–36 km away. No clicks were emitted during the response period, indicating cessation of normal echolocation-based foraging. A sharp decline in both acoustic and visual detections of conspecifics after exposure suggests other whales in the area responded similarly. Though more data are needed, our results indicate high sensitivity of this species to acoustic disturbance, with consequent risk from marine industrialization and naval activity.