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Soundings Archive

Passive Acoustic Monitoring:  Overrated?
There is significant disagreement, debate and uncertainty over what levels of anthropogenic  sound are harmful to marine mammals.  Setting aside this issue for now, any regulatory restrictions on sound will have to be monitored and enforced through some means. Passive Acoustic Monitoring (“PAM”) is widely touted as one tool to achieve this goal, in particular by NGO environmentalists.  Some regulatory bodies share the NGOs’s enthusiasm about PAM.  

According to the International Council for the Exploration of the Sea’s Ad-hoc Group on the Impact of Sonar on Cetaceans and Fish,  PAM  “is the term used for listening passively to sources of sound....” 1  PAM requires observers to use hydrophones or remote or autonomous recording devices to determine the presence of marine mammals by listening for their  vocalizations. For a few species such as sperm whales,  PAM might supplement visual observation during night time operations or when there is concern for submerged  marine mammals.  The United States Department of Interior’s Minerals Management  Service (“MMS”) recently implemented a voluntary experimental PAM study for sperm whales in the Gulf of Mexico for oil and gas seismic operations there.  MMS explained:

“Whales, especially sperm whales, are very vocal marine mammals, and periods of silence are usually short and most often occur when these animals are at the surface and may be detected using visual observers. However, sperm whales are at the greatest risk of potential injury from seismic airguns when they are submerged and under the airgun array. Passive acoustic monitoring appears to be very effective at detecting submerged and diving sperm whales, and some other marine mammal species, when they are not detectable by visual observation. The MMS strongly encourages operators to participate in an experimental program by including passive acoustic monitoring as part of the protected species observer program.” 2

Based on the best data currently available, however, PAM is useless for non-vocalizing species, and it cannot accurately provide the range or depth of any marine mammal from the vessel.  In some studies, PAM did not detect marine mammals when visual observers did. Consequently, unless the technology improves, PAM’s utility for any purpose is questionable, and it could not be used by regulators to demonstrate non-compliance by a vessel with its exclusion zones:  i.e., the area around the vessel which must not include marine mammals during acoustic operations.

PAM’s current limitations have been frequently noted.  For example, MMS’s    programmatic environmental impact statement for oil and gas seismic exploration in the Gulf of Mexico includes a long list of PAM “limitations”:   

    “Passive acoustic monitoring only works for vocalizing whales; manyanimals are quiet much of the time, especiallywhen disturbed; 2) difficult to determine therange to the vocalizing animals when using atowed array; 3) cannot readily determine depthto vocalizing animals; 4) for fixed hydrophones(ship or bottom mounted recorders, sonobuoys,ocean bottom cables), area ofcoverage/detection range may be limited bynoise, requiring more sensors to cover a seismicsurvey area; 5) requires hydrophone arrays betowed behind the survey vessel (or from anadditional chase boat); 6) hydrophoneperformance may be affected by tow speed andthe ship’s acoustic characteristics, limitingdetection range; and 7) cost ramifications.” 3

In addition to the voluntary oil-and-gas PAM program in the Gulf of Mexico, MMS is also testing PAM in the multi-year sperm whale seismic study being conducted in the Gulf of Mexico.  One recent report on this study states,  The work in 2002 and 2003 has shown that a passive acoustic method that uses two or three hydrophones deployed as either a vertical or large-aperture towed array can be used for tracking sperm whale dive profiles.” 4

This statement is difficult to reconcile with other reports.  For example, the National Oceanic and Atmospheric Administration (“NOAA”) and the Navy have recently written on PAM as applied to beaked whales.   The NOAA/Navy article includes an appendix from Lamont-Doherty Earth Observatory (“L-DEO”) that discusses L-DEO’s attempt to use PAM to spot marine mammals in the Gulf of Mexico.  The appendix summarizes L-DEO’s  results as follows, “Three visual sightings were made during periods of acoustic monitoring, but no marine mammals were detected acoustically.”5  

The Navy/NOAA article concludes,  “The utility of passive acoustics detection for beaked whales is extremely limited because, for most species, we do not know what sounds they make.” 6

ACCOBAMS has also noted PAM’s limitations.   ACCOBAMS is the acronym for Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea, and Contiguous Atlantic Area. ACCOBAMS is now reviewing proposed recommendations and guidelines for anthropogenic noise and marine mammals. The report of these recommendations and guidelines includes the following discussion of PAM:

“Passive acoustics has the potential for detecting diving animals, if they produce sounds, and also for accurately establishing direction and distance of the “contact”; but in most of the current implementations the accurate localization is very difficult as repeated contacts with the same source are normally required for a reliable triangulation. In the real world, the triangulation of sources that move fast in three dimensions and don’t vocalize regularly is very difficult. Also, as often animals dive or swim in group it is impossible to correctly triangulate on sounds emitted by a same source. In most cases passive acoustics provides early alerting and gives cues to visual observers. As the estimation of the distance of the animals may be inaccurate it is impossible to establish if an animal is “in” or “out” of the safety area; when visual observations are less effective, as is the case at night or in poor visibility conditions....” 7

Despite these problems with PAM, the ACCOBAMS report recommends its required use. 8     

In April 2004, Scotland’s Joint Nature Conservation Committee published Guidelines for Minimising Acoustic Disturbance to Marine Mammals from Seismic Surveys.  Section 4 of these JNCC Guidelines recommends the required use of PAM:

    “if sensitive species are likely to inhabit the proposed survey location. This additional measure is required where there are species of particular conservation importance or where a given species or group is difficult to detect by visual observation alone.” 9

The JNCC Guidelines recommend the required use of PAM despite acknowledging that “[i]n many cases, PAM is not as accurate as visual observation when determining range.”10

In conclusion, PAM is often viewed as the future technology for detecting the presence of marine mammals during acoustic operations.  In addition to the examples provided above, NOAA has announced that it  “is planning a global passive acoustic monitoring network in collaboration with various NOAA and academic partners.”11  This enthusiasm for PAM is difficult to reconcile with the technology’s current limitations.   In its current state, PAM is neither accurate or reliable for determining whether marine mammals might be harmed by anthropogenic sound. 

            1 Report of the Ad-hoc Group on the impact of Sonar on Cetaceans and Fish, page 30, available online.

            2  Notice to Lessees and Operators of Federal Oil, Gas, and Sulphur Leases in the Outer Continental Shelf, Gulf of Mexico OCS Region, Implementation of Seismic Survey Mitigation Measures and Protected Species Observer Program March 1, 2004), available online.

            3 Geological and Geophysical Exploration for Mineral resources on the Gulf of Mexico Outer Continental Shelf, page S-8 (MMS 2004-054), available online.

            4 Sperm whale Seismic Study in the Gulf of Mexico, Annual Report Year 2, MMS 2004-067,  available online.

            5 The article is available online. The quote is from page 8 of the article.  NOAA’s National Marine Fisheries Service (“NMFS”) did not require L-DEO to use PAM when the agency recently permitted L-DEO to do acoustic research, in part because PAM is ineffective in shallow water.  69 FR 58131,  58138 (Sept. 29, 2004).

            6 Id.,  page 19.

            7 ACCOBAMS, Recommendations and Guidelines To Address the Issue of the Impact of Anthropogenic Noise on Marine mammals in the Mediterranean Sea: Toward a Permit System for the Accobams Area, page 6 (April 28, 2005), available online at online .

            8 Id., pages 16, 18. 

            9 The JNCC Guidelines are available online at  online.

            11 NOAA Magazine, NOAA Studies Sounds in the Sea, available online.


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