Active military sonar is used by navies around the world to communicate, detect objects and vessels, and navigate underwater. Active sonar transmits sound into the water and, as marine mammals also rely on sound for communication, orientation, and navigation, additional ‘noise’ in the environment may impact them. For more information on military sonar, please see www.marinemammalriskmitigation.com. This particular page summarises potential physical effects military sonar may have on marine mammals. For information regarding possible masking effects, please see www.cetaceanmonitoring.org, and for possible behavioural disturbances, please see www.staticacousticmonitoringsystem.co.uk.
PHYSICAL EFFECTS OF NOISE ON MARINE MAMMALS
Physical effects are those that impact an animals’ anatomy directly. If marine mammals are exposed to sufficiently high sound levels, they can be effectively ‘deafened’. This occurs when sensory hair cells are damaged, causing an upward shift in threshold of hearing (i.e. a previously audible sound must be louder to be detected). The effect can be temporary or permanent. Temporary Threshold Shifts (TTS) can last minutes or hours, depending on the level and duration of sound exposure. Permanent Threshold Shifts (PTS) are a result of extremely loud and sudden exposure to damaging sound, or from chronic exposure to very high levels of sound, and as the name suggests, cannot be recovered from.
Mooney et al. (2009) conducted a study on temporary hearing loss of captive bottlenose dolphins (Tursiops truncatus) in relation to Mid Frequency Active Sonar (MFAS) in Hawaii (http://rsbl.royalsocietypublishing.org). The authors used a recorded MFAS signal, with transmissions spaced 24 seconds apart, to mimic normal MFAS application. To assess whether TTS occurred, hearing threshold (using physiological methods of auditory evoked potentials) for a 5.6 kHz tone was measured before and after exposure to the sonar signal. The authors observed TTS only after repeated exposure to the sonar signal with levels 214 dB re 1 μPa2 s or above. Recovery from TTS occurred usually within 20 minutes after exposure, and always within 40 minutes. The control sessions, which had no exposure to sonar or other sounds, produced no changes in hearing threshold. From this experiment it appears that MFAS can induce TTS in bottlenose dolphins, and presumably other odontocete species; however, for this to occur, exposure to high sound levels for an extended period of time are necessary.
Talpalar and Grossman (2005) suggested that TTS in odontocetes during a deep dive may disable their echolocation ability, which would lead to disorientation and likely a much faster ascent, potentially causing decompression sickness (www.researchgate.net). The authors also propose that the effects of sonar noise on deep diving cetaceans may be enhanced due to the high water pressure the animal is under during a deep dive. They suggest that adaptations of the central nervous system to high pressure may give rise to an enhanced startle response. When a high intensity noise alarms these animals, it is likely to cause an erratic flight response, leading to an increased rate of ascent, as well as irregular navigation, which could cause the animal to strand; however, more studies on neural effects of sound waves on diving marine mammals are needed to understand how this combination affects their behaviour.
DECOMPRESSION SICKNESS AND MARINE MAMMALS
Deep diving species appear to be disrupted by high levels of sound (Baird et al. 2006; de Ruiter et al. 2013), and are thought to be susceptible to a similar condition in human scuba divers known as decompression sickness (DCS), which can result in tissue damage and haemorrhaging. This is because animals that dive to extreme depths are on a strict energy budget, and end their dive with just enough oxygen stored to reach the surface. If they become startled by a noise source, they may try to flee from it by swimming more rapidly to the surface. Surfacing rapidly may cause gas bubbles to form in blood and tissue, and can cause arteries to become blocked.
Fernandez et al. (2005) investigated the lesions of beaked whales that mass stranded following naval sonar exercises in the Canary Islands (http://vet.sagepub.com). Fourteen beaked whales from three different species: Cuvier’s (Ziphius cavirostris); Blainville’s (Mesoplodon densirostris); and, Gervais’ (Mesoplodon europaeus), stranded after onset of MFAS, with the first being only four hours after start-up. Of the fourteen that stranded, ten were examined post-mortem and while in good body condition externally, were found to have gas-bubble-associated damage in vessels and tissue of vital organs. The authors found that nitrogen supersaturation of tissues exceeded a tolerable threshold, causing the formation of these bubbles. It is not clear whether sonar caused modification to the diving behaviour, causing excessive supersaturation, or if sonar lowers the threshold for bubbles to form. Either or both of these processes could intensify and maintain bubble growth, or even initiate an embolism. The findings of the examinations are similar to those reported in human DCS. It is these DCS-like symptoms that are presumed to have caused death, or to have caused the whales to strand themselves, and die shortly after with ‘severe cardiovascular failure brought on by “stranding stress syndrome”’ (Fernandez et al. 2005). It appears that exposure to MFAS induces this syndrome and affects deep diving beaked whale species that are living on the physiological edge.
It is clear that far more research is needed in this field to better understand the effects caused by military sonar on marine mammals. With more information, better policies and guidelines (www.marinemammalmitigation.co.uk) can be implemented, governing the use of sonar, and thus providing the required protection for marine mammals and clearer information for the military.