ANALYSIS OF SOUND SPEED MEASUREMENTS FROM ACOUSTIC SOURCES OBSERVED AT PIONEER SEAMOUNT Roger W' Bla

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COMMUNICATION BY BLUE WHALES???
Pioneer Seamount Observatory

• OBSERVATION OF BLUE WHALE CALLS FROM PIONEER
  SEAMOUNT
• Michael D. Hoffman,1 Newell Garfield,2 and Roger
  Bland1
• Physics and Astronomy Department and Romberg
  Tiburon Center for Environmental Studies, San
  Francisco State University2. Department of
  Geosciences and Romberg Tiburon Center for
  Environmental Studies, San Francisco State
  University
• http//www.physics.sfsu.edu/seamount

We have carried out a high-statistics study of
the B call of the blue whale (Balaenoptera
musculus), as observed from the Pioneer Seamount
underwater observatory, 90 km off the California
coast. Data were collected in several contiguous
blocks, via an underwater cable, over a period of
13 months. The continuous operation and the
non-invasive nature of the observations make this
a homogeneous and unbiased sample. We find a
remarkable degree of consistency in the B
calling patterns of the blue whale The calling
interval is constant to 4 accuracy and the
frequency of calls is adjusted to be the same,
over long periods of time, and for all calling
individuals, to an accuracy of 0.5 . The
calling frequency itself is apparently changing
with time, with all individuals visiting Pioneer
Seamount tracking the change. Two substantially
different types of calling sequences are
observed, each at the same frequency, each with
its well-regulated (but different) calling
interval. A number of questions concerning
blue-whale acoustic activity are raised. What
types of behavior correspond to these two calling
sequences? What is the function of the precisely
regulated call frequency? How do individuals of
different size adjust their call to the same
frequency? And what sense is able to perceive
frequency to this remarkable accuracy? And
finally, is there any information transmitted by
acoustic calling which is so remarkably invariant?
Underwater topography near Pioneer Seamount.
(Graphic courtesy of PMEL/NOAA)
Contour map showing the location of Pioneer
Seamount. (Graphic courtesy of the Monterey Bay
National Marine Sanctuary.)
CALLING PATTERNS Other researchers have found
several distinct calling patterns for Eastern
Pacific blue whales. The most prominent in our
data are patterns of repeated A-B call pairs
(AB calling) and sequences of B calls alone
(B-only calling). For spectrograms of these
sequences, see the GALLERY to the far right.
THE BLUE-WHALE A AND B CALLS Blue-whale
sonorization consists mainly of two calls,
referred to as the A call (a pulsing call) and
the B call (a moan of steadily decreasing
frequency.) Other, rarer calls are shown in the
GALLERY to the right.
SEASONAL AND DIEL VARIABILITY
192 sec
128 sec
128 sec
125 sec
177 sec
191 sec
140 sec
136 sec
130 sec
199 sec
131 sec
135 sec
139sec
141
140
128
133
130
197
138
128
137
132
132
136
130
242
183
137
151
138
161
227
128
149
168
190
126
131
143
127
238
183
183
183
183
183
183
183
183
183
183
183
183
A
90 Hz
16 Hz
observatory off line
A?
A?
B?
B?
The A call consists of a series of very regular
pulsations, with two components, at different
frequencies and out of phase in time. The
high-frequency component peaks at 90 Hz and shows
little harmonic structure. The low-frequency
component has most of its power near 16 Hz, the
fundamental frequency of the B call. Most A
calls seem to have short sections before and
after the pulsations sounding very much like the
B call..
The clearest trends in these data are the absence
of calls from mid-April to mid-July, followed by
plentiful calling through mid-November. There
were more calls in 2001 than 2002, and the calls
were fainter in 2002. One simple explanation
would be that the whales stayed closer to shore
in 2002, though we have no other evidence for
this hypothesis. The grayed-out points show
the number of calls triggering the matched-filter
detection. The black points show the number of
calls remaining after least-squares fitting. A
manual scan of spectrograms was carried out in
addition for the 2002 data.
B
48 Hz
harmonics
32 Hz
16 Hz
The B call consists of a repeating waveform of
extremely predictable frequency, about 16 Hz.
The power at multiples of 16 Hz consists of
harmonics of the fundamental the waveform
departs increasingly from a perfect sinusoid as
the power in the harmonics increases. The
harmonic structure, and the envelope of the
signal (power versus time) vary in our data
sample especially in the B-only sequences.
However, the fundamental frequency at the center
of the call and the (slight) rate of decrease of
the frequency during the call are extremely
reproducible, as described below.
THE CONSTANT-FREQUENCY B CALL The B call of
the blue whale is a contact call it seems to be
used by the whales to stay in contact with each
other. This call is remarkable in at least two
respects. (1) All whales tune to the same
frequency, with a pitch discrimination far better
than that of humans. (2) This frequency chosen
by the group has steadily decreased over recent
years. The determination of this frequency is
described below. The average frequency over the
13 month period studied here is 16.01 Hz, with a
variance of the sample of 0.09 Hz. (This
variance may be dominated by measurement error.)
From our data alone we determine a rate of
decrease in this frequency of 0.08 Hz, in rough
agreement with published observations.
Here the hour of the day (UT) is plotted
vertically against the date, horizontally, for
all 6984 triggers from the matched-filter
detection. There is no striking change in the
day-night variation over the 13-month period of
this study.
AB calling T 128 sec, ?8 sec
B-only calling T 50 sec, ?3 sec
Spectrogram obtained from averaging waveforms for
a sequence of loud calls. We have represented
the central 6 seconds of this call by a sinusoid
with frequency decreasing with time. The
waveform from each of the 6984 calls identified
using a matched filter was fit using least
squares to determine its central frequency (f0)
and rate of decrease of the frequency (alpha).
The results are shown to the right.
6 pm PST
6 am PST
Diel variability of the rate of B call
triggers. While calling is observed at all hours
of the day, the plot peaks in those hours
corresponding roughly to the daylight hours in
the Pacific Daylight time zone. However, there
are various possible biases which could lead to
such an effect which we have not so far excluded
(for instance, a day-night difference in the
ambient acoustic noise).
Histogram of time intervals. The main peak
corresponds to the spacing between peaks in AB
sequences, with a peak at twice its value from
cases where an intermediate peak is missed by the
pattern recognition. The lower peak shows the
peak spacing for B-only calling. Both times are
extremely well defined.
Help and advice is gratefully acknowledged from
Dave Mellinger, Chris Fox, Jonathan Klay, Andy
Lau, and Haru Matsumoto, NOAA-PMEL Jim Mercer and
Lyle Gullings, APL, University of Washington
Ching-Sang Chiu, Monterey Naval Postgraduate
School and John Bourg and Jim Lockhart, San
Francisco State University
Time interval between B calls. The time to the
following peak is shown on the x axis, and the
time on to the peak after that, on the y axis.
The peaks are found by automatic pattern
recognition, so many peaks are missing. The
prominent bands show the peak spacing of about
125 sec for AB calling sequences and of about 50
sec, for B-only calling.