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Since
1991, NOAA's Pacific
Marine Environmental Laboratory (PMEL) in Newport, Oregon
and National Marine Mammal
Laboratory Laboratory (NMML) in Seattle, Washington
have collaborated since 1993 on a joint study to assess
the potential of long-range acoustic monitoring of free-ranging
populations of large cetaceans.
NMML brings many years of experience in population stock assessment
based on field observations, with supporting data on habitat,
near-field acoustics and behavior. PMEL brings expertise in
underwater acoustics and access to both the U.S. Navy's underwater
SOund SUrveillance System ( SOSUS)
and autonomous moored hydrophone recorders designed for long-term,
deep-ocean deployment. This joint study has been largely funded
through the
Strategic Environmental Research and Development Program (SERDP),
with additional support from the Office
of Naval Research (ONR), NOAA's
Environmental Services Data and Information Management (ESDIM)
Program, NMML,
and the NOAA VENTS
program at PMEL.
PMEL
has been monitoring and archiving sound recordings from the
U.S. Navy' underwater (SOund SUrveillance
System or "SOSUS") fixed hydrophone arrays since
1991. Although the research was directed to the study of underwater
seismicity and volcanic activity, initial examination
of the data indicated numerous marine
mammal calls. PMEL/NMML joint acoustic research has concentrated
on the low frequency calls of baleen whales, particularly
the blue whale and fin
whale. The most readily detected signals are produced
by the blue whale, which produces sweeping calls over a several
Hertz range (around 17 Hz) with durations of up to sixteen
seconds. The call
of the blue whale in the northeast Pacific is often sufficiently
loud to be detected on more than one hydrophone array. This
distinctive call allows the use of mathematical matched filters
to detect the signal within the ocean's ambient noise and
allow localization of calls to within a few kilometers from
ranges of hundreds of kilometers. This powerful technique
provides a remote means of acoustically surveying thousands
of square kilometers of open ocean for the presence and movements
of large whales.
In
order to acoustically monitor areas of the world ocean not
covered by existing fixed hydrophone arrays, PMEL has developed
autonomous moored hydrophones
(Fox and Matsumoto 1995) to record acoustic energy from
both underwater seismic activity as well as that from whale
calls. The hydrophones are designed to be moored in the oceanic
sound channel (or SOFAR channel, for SOund Fixing And Ranging);
the titanium case containing the data recorder can withstand
pressure to at least 1000 m below sea level. These instruments
are capable of recording frequencies
from 1 - 20,000 Hz, and depending on the sampling rate, can
record data for over a year before servicing is required.
The hydrophones are designed to be deployed as an array of
independent instruments whose geometry can be determined by
the needs of the experimenter in order to localize acoustic
sources of interest. Currently, 6 autonomous hydrophones are
moored in the Eastern Tropical Pacific (ETP), the Gulf of
Alaska and the mid-Atlantic Ocean. Hydrophone locations can
be seen here
This
powerful combination of SOSUS and autonomous moored hydrophone
data has enabled PMEL researchers to record the low-frequency
calls of blue and fin whales throughout the Pacific Ocean,
and to identify regional differences in blue and fin whale
vocalizations. Locating calling whales also enables PMEL to
identify apparent seasonal shifts in whale distributions.
Correlating these data with NMML current field observations
and their extensive historical database of species distributions
may help answer critical population and stock management questions.
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