Underwater military SOund Navigation And Ranging (SONAR) systems (http://en.wikipedia.org/wiki/Sonar) are used by the navy to navigate, communicate, and detect objects and other vessels on or under the surface of the water using sound propagation. There are two forms of military sonar: passive and active. Passive sonar involves listening to received sounds without transmitting any sound. Active sonar involves transmitting sounds into the water, then interpreting received echoes. It is important that the two are not confused.
Sonar research increased vastly during World War I due to the importance of detecting enemy submarines. British scientists developed hydrophones, but a French physicist, working alongside a Russian electrical engineer, experimented with active sonar. In 1918 both teams had built prototype active systems, with the British system known as ASDIC (derived from Anti-Submarine Division), deployed on vessels in 1923 and utilised during World War II. The British transferred ASDIC technology to the US during World War II to continue research. During this period it obtained its current appellation ‘sonar’. Sonar research continues to this day, in many different nations, both for military and civilian uses.
There are different levels of military active sonar in use today, based on frequencies in which they operate, and these are classified broadly into three ranges: low, mid, and high frequency active sonar. Different navies utilise different frequency ranges for active sonar, with general distinctions, described by D’Amico et al. (2009).
LOW FREQUENCY ACTIVE SONAR
Low Frequency Active Sonar (LFAS) operates mostly in the range of 600–1,500 Hz, with the exception of the U.S. Navy’s SURveillance Towed Array Sensor System (SURTASS, en.wikipedia.org/wiki/Surveillance_Towed_Array_Sensor_System), which operates between 100 – 500 Hz (Hildebrand 2009). In deep water unconstrained by topographical features (such as narrow canyons), LFAS sound is able to propagate over long distances.
MID AND HIGH FREQUENCY ACTIVE SONAR
Mid Frequency Active Sonar (MFAS) operates between 3 – 14 kHz. High Frequency Active Sonar (HFAS) operates > 14 kHz, and has a much shorter transmission range than LFAS when comparable source levels are used. These ranges differ slightly to those defined by the U.S. Navy, which describes LFAS as < 1 kHz, MFAS as 1 – 10 kHz, and HFAS > 10 kHz (U.S. Navy 2008).
POTENTIAL EFFECTS OF MILITARY SONAR ON MARINE MAMMALS
The production and detection of sound is of utmost importance to marine mammals and it follows that significant levels of man-made (anthropogenic) sound in the ocean has the potential to influence routine behaviours, or in severe cases, cause auditory impairment (see www.marinemammalseismic.com for information on how seismic exploration sound may affect marine mammals).
Active military sonars are reported by some biologists to be capable of producing sound Source Levels (SL) of 220 – 230 dB re 1 μPa @ 1 m (Cox et al. 2006) – though levels are often disputed, and information can be misreported by many stakeholders because of the strong emotion associated with concern to marine mammals. The U.S. Navy, however, states slightly higher levels than this, providing common SL of 235 dB re 1 μPa @ 1 m for MFAS (U.S. Navy 2008). The frequency and SL used is often dictated by the purpose of the sonar use. A review by Richardson et al. (1995), states that sonar used for searching have lower frequencies (2 – 57 kHz), but higher SL (above 230 dB re 1 μPa @ 1 m ), than those used for navigation and obstacle avoidance (25 – 500 kHz and above 220 dB re 1 μPa @ 1 m). With source levels such as these, under ideal conditions, sound is able to propagate over long distances and therefore has potential to impact marine mammals over wide areas. There are numerous reasons why marine mammals may be affected by anthropogenic sound in the ocean; these can be divided into the following categories: behavioural disturbance, masking, and physical effects.
Behavioural disturbance effects of noise on marine mammals
Behavioural disturbance is characterised largely by avoidance behaviour. This is evident when an animal avoids a noise source by making changes to its migration, feeding, or social behaviour. Such changes could be detrimental to marine mammal species if the population depends on a specific site or habitat for survival, such as feeding or breeding grounds, especially if the disturbance is long lasting. For example, Blainville’s beaked whales (Mesoplodon densirostris) have been found to cease echolocating and move away from the source of a MFAS signal by Tyack et al. (2011). For a more comprehensive explanation of this paper, and others like it, please see www.staticacousticmonitoringsystem.co.uk.
Masking effects of noise on marine mammals
Masking occurs when an anthropogenic sound source obscures sounds of interest for a marine mammal, potentially hindering communication with conspecifics in a pod, between mother–calf pairs, or missing tell–tale sounds of an approaching predator, rendering individuals more vulnerable to attack. A study by DeRuiter et al. (2013a) found that false killer whales (Pseudorca crassidens) whistled more frequently during MFAS playback conditions, compared to before or after playback, presumably to counter the masking effect of the sound. For more information on the masking effects of sonar on marine mammals, please see www.cetaceanmonitoring.org (In the section entitled: ‘pilot and false killer whales and masking from military sonar’).
Physical effects of noise on marine mammals
Physical effects are those that impact the anatomy of the animal directly. If marine mammals are exposed to sufficiently high sound levels, they can be effectively ‘deafened’. This is known as a Threshold Shift (TS) and can be temporary or permanent. Mooney et al. (2009) conducted a study into TS on bottlenose dolphins (Tursiops truncatus) in Hawaii, showing that MFAS can cause temporary TS. There is also evidence that deep diving species startled by a noise source can develop a similar condition to decompression sickness in humans, which can then lead to tissue damage (DeRuiter et al. 2013b). For more detailed information on these papers, and others like them, please see www.acousticmonitoring.co.uk.