PASSIVE ACOUSTIC MONITORING


backgroud acoustics

 

Background

Ocean background noise at different frequencies, as measured by Wenz (1962). Sound levels are given in dB re 1 µPa2/Hz. Reprinted from National Research Council of the National Academies (2003) Ocean Noise and Marine Mammals. National Academies Press, Washington, D.C. 204 p. 

People and marine animals use sound in the sea to accomplish many tasks. Since light travels relatively short distances in the ocean, sound is the primary means by which many marine organisms convey and sense information over biologically-relevant spatial scales.  The oceans are filled with sound generated by a variety of natural sources, including not only biotic sources (e.g. marine life) but abiotic sources such as breaking waves, earthquakes, wind and rain.  Underwater sound is also generated by a variety of anthropogenic sources, such as vessels, military sonars, oil and gas drilling, and some oceanographic research technologies. The background “omnipresent” sound in the ocean is called ambient noise. The primary sources of ambient noise vary with frequency (pitch or tone).  For example, ambient noise between 20-500 Hz is primarily generated by vessels whereas ambient noise between 500-100,000 Hz is mostly due to spray and bubbles associated with breaking waves. For more information on sources of sound in the ocean see Discovery of Sound in the Sea (http://www.dosits.org).

The steady (and ever increasing) intensity of background ocean noise in many (particularly coastal) areas has received growing attention and concern among scientists, policy-makers and the public. Research suggests that increased background noise and specific sound sources might impact marine animals in several ways, with potential impacts including alterations in behavior, masking (preventing marine animals from hearing important sounds), and physical injury in the form of temporary or permanent hearing loss or tissue damage. The status of current research regarding the impacts of sound on marine life is summarized by the Discovery of Sound in the Sea (http://www.dosits.org) as well as in several review articles and reports referenced therein.

In addition to direct or indirect impacts to species, noise interferes with human uses of passive acoustic techniques to observe, and thus better understand and effectively utilize while conserving, natural environments.  Acoustic sensing techniques provide an optimal means for detecting and characterizing physical and biological features of ocean areas. In particular, passive acoustic technologies are able to provide enhanced and unique scientific data on living marine resources, biotic and abiotic characteristics of marine ecosystems, and the effects of anthropogenic sound on marine species and their ecosystems.  Passive acoustic applications are currently undergoing a technological revolution and “coming of age”, thereby providing powerful and versatile alternatives to existing technologies and applications for managing and conserving marine ecosystems. 

The SBNMS is home to many vocally-active marine species that are protected and/or managed by NOAA under multiple US statutes, including the National Marine Sanctuaries Act, the Marine Mammal Protection Act, the Endangered Species Act and the Magnuson-Stevens Fisheries Conservation and Management Act.  The SBNMS is also a busy place for human commerce, and thus is subjected to high levels of sound-producing activities.  Meeting marine resource protection and management objectives in the SBNMS necessitates understanding of the relative inputs of sound sources within the sanctuary and the possible effects of these sounds on marine animal behavior.  Passive acoustic monitoring and research in the SBNMS aims to educate stakeholders regarding the importance of listening underwater, both for marine wildlife and for humans, and to highlight the role that marine protected areas are playing and/or could play in managing ocean noise.

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