PASSIVE ACOUSTIC MONITORING
Autonomous
Recording Unit (ARU) developed by Cornell University's Bioacoustics
Research Program and used to monitor underwater sound in
the SBNMS. |
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People
and marine animals use sound in the sea to accomplish many
tasks. Since light travels relatively short distances in
the ocean, sound is often used for such basic activities
as finding food or a mate, navigating, and communicating.
For that reason, the oceans are filled with sound generated
by a variety of human-related sources, including not 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 the frequency. 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.
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Current
knowledge about the effects of sound on marine animals relies
heavily on experimentation with small numbers of individuals in
captivity and/or post-hoc evaluation of mortality events in the
wild, in which cause and effect is often impossible to determine.
Due to their charismatic nature, protected status and their use
of sound for communication, marine mammals have been the focus
of increasing levels of attention and controversy possible adverse
effects of noise in the marine environment. Marine mammals have
been shown to manifest behavioral changes in the presence of certain
types of noise. In addition, exposure to anthropogenic noise can
impact cetaceans by masking biologically important sounds (e.g.
intraspecific communication and localization of prey resources),
provoking avoidance or attraction, causing temporary or permanent
hearing damage and, in extreme cases, death.
In
its 2003 report (one of three devoted to sound sources and marine
mammals), the US National Research Council of the National Academies'
(NRC's) Committee on the Potential Impacts of Ambient Noise in
the Ocean on Marine Mammals concluded that concern surrounding
anthropogenic sound and marine mammals was warranted, given the
endangered status of many marine mammals, the identified importance
of sound in the lives of marine mammals, the potential for harm
from excessive noise and the paucity of data with regards to the
amount of sound introduced into the oceans by human activity and
its potential impact on marine mammals.
The
NRC's report recommended the establishment of "noise budgets",
defined as the sum of the relative contributions made by identified
sound sources to the total sound field. The report further recommended
that "noise budget" determinations for various parts
of the ocean should include representations of seasonal and spatial/temporal
differences. Finally, the NRC specifically identified the need
to define the sound contribution of different vessel types within
the major category of shipping. While the report's focus was global,
many of its insights and recommendations can be used at the sanctuary
level to provide a local understanding of the issue. Insights
achieved at the local level can then be used to inform the larger
issue at national and international levels.
Numerous
anthropogenic sources of underwater sound produced both within
and in the waters surrounding SBNMS contribute to the sanctuary's
ambient noise budget. Commercial, recreational, military, and
research vessels all contribute to ambient underwater noise in
the sanctuary, whether directly through their marine operations
(e.g., engines, props and electronics) or indirectly through the
activities they perform (e.g. towing and dredging). Whales are
known to aggregate in and near the existing traffic separation
scheme (i.e., shipping lanes to and from the Port of Boston) and
their long-term acoustic exposure to vessel traffic may represent
a source of chronic impact.
The
operations of fishing vessels regularly overlap the distribution
of cetaceans in the sanctuary and may be an additional source
of repeated acoustic disturbance. In addition, some vessels, such
as commercial and private whale watching boats, preferentially
expose large whales to noise as a byproduct of routine and frequent
close approaches, creating another opportunity for chronic exposure.
Finally, because low-frequency sounds can retain their intensities
over large distances, industrial and commercial activities taking
place or proposed within the Gulf of Maine, can or will contribute
to low frequency sound in the sanctuary.
To
begin characterizing the acoustic environment and health of SBNMS,
a collaborative group of researchers from the SBNMS, NOAA Fisheries
(Northeast Science Center and Regional Offices) and Cornell University's
Bioacoustics Research Program has installed an array of nine autonomous
recording units (ARUs) to monitor underwater sounds between 0
and 1000Hz throughout sanctuary waters for one calendar year.
ARU
data will be used to compare sanctuary sound levels with (1) baleen
whale audiogram models to estimate the potential for hearing loss
and (2) the sounds produced by baleen whales in the sanctuary
to determine the potential for masking. To calculate the "noise
budget" of SBNMS, the contributions and frequency characteristics
of noise from biological, meteorological, and anthropogenic sources
will be identified. Modeling will determine, for a specific location
and its acoustic environment, the range at which a sound of a
given frequency could be received from that location and how that
range would change daily, seasonally, and yearly, as a result
of changes in the acoustic environment.
ARU
data will be used to localize vocalizing whales (identified by
the species-specific characteristics of their vocalizations) and
calculate distributions and acoustic densities for different species
in sanctuary waters throughout the year. These data will aid the
sanctuary's ongoing monitoring of whale densities relative to
vessel traffic. Due to their critically endangered status, locating
vocalizing right whales within the sanctuary will remain a priority
for ARU data analysis.
The same data can be analyzed to determine the number of vessels
traversing the acoustic monitoring area, the acoustic characteristics
of individual vessel types, and the received characteristics of
the vessel as a function of such factors as distance, bathymetry,
and water temperature.
Financial
support for this research is provided by the National Marine Sanctuary
Program, NOAA Fisheries Northeast Science Center and Northeast
Regional Office, the National Marine Sanctuary Foundation, the
International Fund for Animal Welfare and the Massachusetts Environmental
Trust
Autonomous
Recording Units (ARUs) in their packing cases onboard the deploying
vessel.
Research design for ARU array affording approximately 85% acoustic
reception coverage with the SBNMS' boundaries.
24 Hour Spectrogram of sound within a portion of the SBNMS resulting
from 2004 pilot study. Note chronic loud sound in the <1000
Hz bandwidth used by baleen whales. Figure credit: Ingrid Biedron,
Bioacoustics Research Program, Cornell University.
PRESENTATIONS
Hatch, L. T., and Wiley, D. 2006. The Gerry E. Studds Stellwagen
Bank National Marine Sanctuary as a model for integrating platforms
to characterize the marine acoustic environment. NOAA Passive
Acoustics Workshop, Woods Hole, Massachusetts; April 12-14.
Hatch,
L. T., Gontz, A. M, Clark, C. W., and Wiley, D. 2006. The Gerry
E. Studds Stellwagen Bank National Marine Sanctuary as a regional
case study for integrating protected species and protected area
management tools to study and mitigate impacts of anthropogenic
noise sources on marine mammals. SC/58/E2. Presented to Environmental
Concerns Working Group's Pre-Meeting to Review the Potential Impacts
of Seismic Surveys on Cetaceans; 58th Annual Meeting, Scientific
Committee of the International Whaling Commission, St. Kitts,
West Indies; May 24-25.
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