Noise Pollution

We rarely think of noise as a form of pollution. But for hearing-sensitive marine creatures, whose sole source of communication over large distance is sound, the noise created by underwater explosions, ship engines, and military sonar can be devastating. Consider the sperm whale – a largely solitary animal that must find its mate by communicating low frequency sounds at depths. (At some "channels" in the deeper ocean, these signals can travel across entire ocean basins.) When anthropogenic noise clogs this channel, the whales cannot find one another to socialize and breed. Other cetaceans use sound to locate prey, and ship noise can prevent them from doing so. Such animals are so sensitive to noise that the cacophony of white noise that now exists in many parts of the ocean has deprived them of sleep and caused what appears to be psychological stress. And even many fish species that communicate with sound are suffering from an increasingly noise-polluted ocean.

Because noise is increasingly widespread and intense in ocean waters, much scientific attention has focused on it, but only very recently. In a chapter of a book on cetaceans in crisis, John Hildebrand writes about the impacts of anthropogenic sound on marine mammals: " …mounting evidence suggests that high intensity anthropogenic sound from sonar and airguns leads to subsequent strandings and mortality of beaked whales…A more pervasive, yet subtle, problem may be the effects of increases in background noise levels from commercial shipping."¹⁷

Most sources of anthropogenic noise occur in continental shelf areas that are the most important habitats for marine mammals and coastal fishes. The U.S. National Research Council has stated that for 119 species of marine mammals and for many non-mammalian marine species as well, hearing and echolocation is the single most important sense for foraging, communicating, and navigating¹⁸. The sound environment is an important aspect of marine mammal habitat, and yet it has only recently been the focus of research determining the ecological requirements of these highly valued, and in many cases, highly threatened species.

Anthropogenic noise may be subdivided into two sorts of threats: acute noise that can cause mortality, hearing impairment, or other sorts of physiological stress; and chronic lower level noise which can also cause physiological stress but can do further damage by interfering with sound-based navigation, communication, and foraging. Noise travels very efficiently in water – nearly 5X the speed of sound traveling in air¹⁹. Sound waves bouncing off the sea surface or sea bottom cause sound to reverberate, adding to the increasing levels of white or background noise in the oceans.

Sound propagation; credit: www.mathinfo.u-picardie.fr

Acute noise has been shown to cause mortality and acute morbidity in many different species of marine animals. Sources of acute noise include active military sonars, underwater explosions such was those occurring in dynamite fishing, ship shock trials, or underwater construction, and air gun array blasts used in seismic surveys. Such airgun arrays are towed behind the approximately 90 seismic vessels in operation today (20% of which are operating at any given time in the ocean), and they can fire every 10 seconds for days at a time. A low-frequency seismic array can generate 215 decibels of sound, comparable to a twin-engine fighter jet at takeoff. Necropsies on toothed whales that mass stranded after being in the vicinity of sonar-using military vessels and seismic vessels have shown ruptured earbones, hemorrhages in fatty tissues of the head, and air bubbles in the lungs and brain²⁰. These animals have particularly acute hearing, and all sound-sensitive organs are affected by acute noise, especially those occurring at mid-frequencies (as is the case with the most damaging military active sonars).

Military active sonar emits loud sound waves that sweep across the ocean to reveal enemy submarines and other objects of security interest. Some mid-frequency sonar systems can emit 235 decibels, sound as loud as a rocket at launch. Even 100 miles from a low frequency sonar system, sound levels can approach 160 decibels, well beyond the Navy's own safety limits for humans²¹. And with a concerted move to perform more military exercises in the shallow waters that are home to many marine mammals, concern for the animals’ welfare is growing.

It should perhaps come as no surprise that all the mass strandings of beaked toothed whales such as beaked whales, dolphins, and pilot whales that have ever been recorded have coincided with large scale naval exercises in the vicinity, including mass strandings in the Canary Islands (Spain), Bahamas, Ionian Sea (Mediterranean) and the southeast Atlantic coast of the United States.

Chronic sounds seem to generate less media attention but can cause enormous problems for animals that use sound to travel, feed and communicate. Ambient or chronic sounds are primarily created by the enormous numbers of vessels now plying the seas, by offshore wind farms, and by the long term operation of seismic airgun arrays.  Such sounds are generally low frequency (but they can be of high intensity) and are prolonged; the most affected species seem to be baleen whales such as fin whales, sei whales, minke whales, and humpback whales. Many of us are familiar with the hauntingly beautiful and complex songs sung by humpback and other whales; less well known (and less admired!) are the sound communications made by coral reef fishes. These communications can be drowned out by the white noise generated by human activity.

Photo credit: www.ead.ae 

But how such anthropogenic noise actually kills, maims or impairs marine animals is not clear²².  A U.S. National Research Council report issued in 2005 entitled, "Marine Mammal Populations and Ocean Noise: Determining When Noise Causes Ecologically Significant Effects" presents a conceptual framework for understanding how noise can cause biologically significant effects like slowing growth, reducing survival, or lowering reproductive rates.  In response to this call for research, and that offered by the U.S. Commission on Ocean Policy report, the National Oceanic and Atmospheric Administration of the U.S. is now proposing to expose marine mammals to loud sonar sounds and monitor their behavior. NOAA Fisheries' Office of Science and Technology has applied for a scientific research permit under the U.S. Marine Mammal Protection Act of 1972 and with Bahamian authorities to conduct these behavioral response studies with an international team of co-investigators²³.

In addition to undertaking more targeted research, there are some concrete steps being taken to try and tackle this large, amorphous, and difficult to address threat. A meeting of marine mammologists and conservationists sponsored by Dokumente des Meeres was held in the Canary Islands in June 2007 to explore the notion of establishing noise-buffered marine protected areas for beaked whales and other noise-sensitive species. The Canary Islands already has a moratorium on military sonar in its territorial waters – the result of a series of mass strandings that coincided with military exercises in the area. In the Alboran Sea off Spain’s Mediterranean coast, the International Maritime Organization has agreed to move shipping lanes in order to protect resident Alboran dolphins²⁴. And in the U.S., shipping traffic within the Stellwagen Bank National Marine Sanctuary has been rerouted to areas within the Sanctuary less frequented by whales. But much more needs to be done.

According to the Millennium Ecosystem Assessment²⁵, toxics loading (in part caused by ship disposal), alien species invasions and noise will all further stress already degraded coastal ecosystems and may impede natural recovery and managed restoration. Human health and well-being will suffer as a consequence, unless the ‘exporting pollution’ problem is addressed and dramatic improvements to coastal management are systematically made across wide regions of the globe.

¹⁷ Hildebrand, J. 2007. Impacts of anthropogenic sound. Pp100-123 In Conservation Beyond Crisis J.E.Reynolds III, W.F. Perrin, R.R. Reeves, S. Montgomery and T.J. Ragen (eds.), The Johns Hopkins University Press, Baltimore, MD.
¹⁸ National Research Council 2003. Ocean Noise and Marine Mammals. National Academy Press, Washington DC
¹⁹ Weilgart, L. 2005. Underwater noise: death knell of our oceans? Accessed on http://www.terranature.org/oceanNoise_Weilgart.htm
²⁰ Hildebrand, J. 2007
²¹ www.ens-newswire.com/ens/oct2005/2005-10-19-07.asp
²² National Research Council. 2003. Ocean Noise and Marine Mammals. National Academy of Sciences Press, Washington DC
²³ www.ens-newswire.com/ens/jun2007/2007-06-08-09.asp
²⁴ P.Popham 2007. Shipping lanes moved to boost dolphin numbers
²⁵ Millennium Ecosystem Assessment. 2005. Vol 1 Current state and trends. Coastal Systems. Pp513-549 In Ecosystems and Human Well-Bring. Island Press, Washington, DC