Ocean Matters

The Development and Impact of
Low Frequency Active Sonar

Sonar Threatens Marine Mammals

Cetaceans (whales, dolphins and porpoises) use sonar to communicate, reproduce and feed in the world’s oceans.  Ironically, human-made sonar technologies threaten to compromise the ability of these sensitive creatures to make a living.  We examine sonar technologies and their potential impacts on cetaceans and other marine mammals.  We also highlight a few solutions.

Many marine mammals use echolocation by emitting sonar clicks. The farther away an object is, the more time it takes for the echo or sonar click to return, allowing the marine mammal to determine the distance between itself and a variety of things such as potential food sources, enemies and shelter. The use of echolocation allows marine mammals to plot a course and feed in deeper or darker waters.  Scientists estimate that echolocation has been used as a sophisticated communication system by dolphins and whales for thousands of years.

Humans, on the other hand, have only used sonar since around 1914 when the first echo sounders were developed for military purposes. In more recent times, the United States (US) Navy has used sonar to detect submarines in oceans around the world. Sounds are emitted across the ocean and bounce back when they hit an object. The lower the frequency of the sounds, the further they travel. At present, mid-frequency active sonar (MFA) is in widespread use. More recently, the US Navy developed a device called Low Frequency Active (LFA) sonar, which was approved for limited use following a lengthy legal process settled in 2003 (although several appeals have continued to be heard in US courts). Low Frequency Active sonar was designed to detect foreign vessels such as submarines in the world’s oceans. The LFA sonar system works by sending waves of low frequency sound or 'pings' into the ocean waters. If these sound pings intersect an object, they will rebound back to the source ship.  The LFA sonar system is primarily employed for defence purposes such as searching for foreign submarines.

The problem with sonar technologies is that they have the ability to injure marine mammals because of high sound levels. High levels of sound can cause some marine mammals to divert from their normal migration paths, disrupt important animal behaviours and lead to increased incidences of stress and panic. Humpback whales, for example, have been shown to stop vocalizing when they come into contact with sonar, which can severely compromise their ability to reproduce. The United States navy originally planned to operate the LFA sonar system in about 75 percent of the world's oceans, although that number has been scaled back due to public pressure and through a lengthy legal process.

There is plenty of evidence suggesting that sonar systems can be fatal to whales. In 2000, 16 whales from three different species were stranded on a beach in the Bahamas while active sonar tests were occurring nearby. All of the whales except one showed evidence of exposure to sonar noise pollution with haemorrhaging around the ears. Although the US Navy has taken steps towards reducing the chances of accidental cetacean deaths, similar unfortunate accidents have continued to occur.  

As a proactive measure, the US navy has decided that it will not operate the sonar system within 22 kilometres of shore, which will help to reduce the chances of marine mammal and human accidents. The sonar system will also not be operated in biologically important areas such as the northeast coast of the United States and Canada, the Costa Rica dome, the Antarctic Convergence Zone and other hotspots (although geographic restraints to testing have been challenged in the courts). There is also a process to nominate other areas that are biologically significant to marine mammals within the framework of the United States National Marine Fisheries Service (NFMS).

Despite these precautionary measures, marine mammals are still vulnerable should they happen to encounter human-derived sonar. That is because the US Navy has determined that an exposure of 180 decibels is not harmful to marine animals.  According to researcher Ben Wilson, however, 180 decibels is the maximum amount of sound before any physical harm occurs to cetacean species. The 180-decibel isoline occurs at 1.0 kilometre or 0.54 nautical miles from the sonar array. If there are any marine mammals detected within the 180-decibel isoline the navy will not operate the sonar system. However, once the animals are within the proper safety zone the sonar will be reactivated. To ensure detection of marine mammals before they are in the danger zone the Navy will use highly advanced fish finding sonar, passive sonar to listen for cetaceans and even visual detection of cetaceans. Even with all of these safety measures there still exists a chance for problems as detection measures are effective only 80 percent of the time. If any cetaceans remain undetected within the 180-decibel exposure limit, they could receive the full 215-decibel exposure, which in the best case scenario will cause hearing damage.

Positive measures that may be taken to reduce the chances of sonar-related marine mammal accidents include, banning the use of LFA sonar in important marine mammal habitats; limiting sonar use throughout migration routes and seasonal feeding or breeding grounds; and employing less harmful passive sonar technology to listen for marine mammals, ensuring none are around prior to switching to more harmful LFA sonar.

Since the use of sonar systems has the potential to affect animal health across the planet, there is an onus on us to properly understand their impacts and take necessary actions to ensure the well-being of marine mammals.

For more information go to:

Natural Resource Defence Council
- Protecting Whales from Dangerous Sonar

US Navy - Marine Mammals and Sound

BBC News- 'Sonar may cause whale deaths'

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