Sounds from airguns and fin whales recorded in the mid-Atlantic Ocean, 1999-2009

Abstract

Between 1999 and 2009, autonomous hydrophones were deployed to monitor seismic activity from 16° N to 50° N along the Mid-Atlantic Ridge. These data were examined for airgun sounds produced during offshore surveys for oil and gas deposits, as well as the 20 Hz pulse sounds from fin whales, which may be masked by airgun noise. An automatic detection algorithm was used to identify airgun sound patterns, and fin whale calling levels were summarized via long-term spectral analysis. Both airgun and fin whale sounds were recorded at all sites. Fin whale calling rates were higher at sites north of 32° N, increased during the late summer and fall months at all sites, and peaked during the winter months, a time when airgun noise was often prevalent. Seismic survey vessels were acoustically located off the coasts of three major areas: Newfoundland, northeast Brazil, and Senegal and Mauritania in West Africa. In some cases, airgun sounds were recorded almost 4000 km from the survey vessel in areas that are likely occupied by fin whales, and at some locations airgun sounds were recorded more than 80% days/month for more than 12 consecutive months.

Full text

Sounds from airguns and fin whales recorded in the mid-Atlantic
Ocean, 1999–2009
Sharon L. Nieukirk
a)
and David K. Mellinger
Cooperative Institute for Marine Resources Studies, Oregon State University and NOAA Pacific Marine
Environmental Laboratory, 2030 SE Marine Science Drive, Newport, Oregon 97365
Sue E. Moore
NOAA Fisheries Office of Science and Technology, 7600 Sand Point Way NE, Bldg. 3, Seattle,
Washington 98115-6349
Karolin Klinck and Robert P. Dziak
Cooperative Institute for Marine Resources Studies, Oregon State University and NOAA Pacific Marine
Environmental Laboratory, 2030 SE Marine Science Drive, Newport, Oregon 97365
Jean Goslin
UMR 6538, Domaines Oce´aniques, Institut Universitaire Europeen de la Mer, Universite´ de Bretagne
Occidentale, Technopole Brest-Iroise, 29280 Plouzane´ Cedex, France
(Received 16 February 2011; revised 24 October 2011; accepted 16 November 2011)
Between 1999 and 2009, autonomous hydrophones were deployed to monitor seismic activity from
16Nto50
N along the Mid-Atlantic Ridge. These data were examined for airgun sounds
produced during offshore surveys for oil and gas deposits, as well as the 20 Hz pulse sounds from
fin whales, which may be masked by airgun noise. An automatic detection algorithm was used to
identify airgun sound patterns, and fin whale calling levels were summarized via long-term spectral
analysis. Both airgun and fin whale sounds were recorded at all sites. Fin whale calling rates were
higher at sites north of 32N, increased during the late summer and fall months at all sites, and
peaked during the winter months, a time when airgun noise was often prevalent. Seismic survey
vessels were acoustically located off the coasts of three major areas: Newfoundland, northeast
Brazil, and Senegal and Mauritania in West Africa. In some cases, airgun sounds were recorded
almost 4000 km from the survey vessel in areas that are likely occupied by fin whales, and at some
locations airgun sounds were recorded more than 80% days/month for more than 12 consecutive
months. V
C2012 Acoustical Society of America. [DOI: 10.1121/1.3672648]
PACS number(s): 43.30.Sf, 43.80.Ka [WWA] Pages: 1102–1112
I. INTRODUCTION
Passive acoustic surveys have become an effective
means of monitoring both the natural and anthropogenic
contributions to ambient noise levels in the world’s oceans.
Autonomous and cabled hydrophones are now used widely
to study the sounds generated by undersea earthquakes, ice
noise, and marine animals. Research has also confirmed
that low-frequency (<1000 Hz) human sources of noise pol-
lution have dramatically increased over the last 50 years
(Andrew et al., 2002;McDonald et al., 2008). The primary
sources of low-frequency anthropogenic noise are the sounds
associated with shipping, military and research activities,
and oil and gas exploration and development (Richardson
et al., 1995;Croll et al., 2001;Hildebrand, 2009). Of grow-
ing concern is the effect these increasing levels of low-
frequency noise have on protected species, such as baleen
whales that are acoustically sensitive and use low-frequency
sound for communication and possibly navigation or prey-
finding (Richardson et al., 1995;Clark et al., 2009). In
particular, the sounds from airgun surveys have been the
focus of several recent marine mammal investigations (e.g.,
Di Iorio and Clark, 2010;Madsen et al., 2006;Weir,
2008a,b), and the potential and observed effects have been
reviewed [e.g., National Research Council (NRC), 2003,
2005;Gordon et al., 2004;Bradley and Stern, 2008]. To
assess the potential effects of airgun sounds on whales, the
temporal and geographical occurrence of this sound and the
distribution of species that are potentially impacted must be
described.
In 1999, a consortium of U.S. investigators deployed an
array of autonomous hydrophones (Fox et al., 2001) to moni-
tor seismic activity along the Mid-Atlantic Ridge (Smith
et al., 2002;Dziak et al., 2004). Although this experiment
was designed to monitor the low-frequency signals of earth-
quakes, the instruments were also capable of recording the
low-frequency calls of several species of baleen whales, as
well as anthropogenic sounds such as ship noise and seismic
airgun pulses. These instruments were located within poten-
tial migratory routes for fin (Balaenoptera physalus) and
blue (B. musculus) whales and were in a remote region
that rarely if ever is included in marine mammal surveys
(Mellinger and Barlow, 2003).
a)
Author to whom correspondence should be addressed. Electronic mail:
sharon.nieukirk@noaa.gov
1102 J. Acoust. Soc. Am. 131 (2), February 2012 0001-4966/2012/131(2)/1102/11/$30.00 V
C2012 Acoustical Society of America
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In a previous study, Nieukirk et al. (2004) found that
sounds from both airguns and baleen whales were recorded
at these mid-ocean sites. Here we update our previous work,
expand our study area, and characterize the seasonal and
interannual variability in airgun sounds in what is now a ten-
year acoustic dataset collected from waters near the Mid-
Atlantic Ridge. To address potential masking of marine
mammal sounds, we also examined the acoustic record for
the 20 Hz pulse calls of fin whales (Watkins et al., 1987).
We chose this species because fin whale vocalizations are
typically plentiful in acoustic records from the Atlantic
(Watkins et al., 1987;Clark and Gagnon, 2002;Nieukirk
et al., 2004), and because fin whale 20 Hz vocalizations are
short pulsive calls that have the potential to be masked by
airgun sounds.
II. BACKGROUND
A. Airgun sounds
Marine seismic surveys are a major source of anthropo-
genic sound in many of the world’s oceans (Hildebrand,
2009). Seismic surveys are conducted primarily in the pur-
suit of oil and gas reserves (Caldwell and Dragoset, 2000),
but research institutions also use this technology to explore
the complex geology of the seafloor. Sounds generated by
seismic airguns are low-frequency (2–188 Hz at the source),
short-duration (<0.1 s), high amplitude (216–261 dB p-p re
1lPa @ 1 m) pulses that are produced as pressurized air is
suddenly released from the airgun cylinders into the water
(Parkes and Hatton, 1986;Richardson et al., 1995;Dragoset,
2000). The expansion of this released air and the following
contraction and re-expansion of this air mass creates a loud
seismic pulse that can be used to image the seafloor and the
rock layers below. The resulting seismic pulse refracts and
reflects off subsurface seafloor structures and is received by
hydrophone streamers towed behind the survey vessel. Air-
guns are fired every 10–60 s for days or weeks at a time,
with occasional interruptions for such actions as turning the
ship that tows the airgun array. Although seismic airgun
arrays are designed to direct the majority of emitted energy
downward toward the seafloor, their sound emission horizon-
tally is also significant (NRC, 2003;Madsen et al., 2006).
B. Fin whale sounds
The sounds produced by the fin whale are among the
best-studied marine mammal vocalizations (Thompson
et al., 1979;Watkins, 1981;Watkins et al., 1987;Edds,
1988;Thompson et al., 1992;Clark et al., 2002;Hatch and
Clark, 2004). Although fin whales produce numerous
sounds, the highly stereotyped, short (0.5–1.0 s) downsweep-
ing sound in the 18–25 Hz frequency band known as the
“20 Hz pulse” is the most common (Watkins et al., 1987;
Hatch and Clark, 2004). Series of pulses occur in long, pat-
terned, song-like sequences with regular interpulse spacing
that changes with geographic location and possibly with
time (Cummings et al., 1986;Watkins et al., 1987;Thomp-
son et al., 1992;Clark et al., 2002;Hatch and Clark, 2004;
Castellote et al., 2011). Current evidence indicates that this
is a male breeding display, as only males have been identi-
fied making these sounds (Croll et al., 2002). Thus, acoustic
surveys of fin whale patterned sequences are likely to detect
only males, but we assume that such survey results are
approximately representative of the relative numbers of all
whales in an area. Fin whale calls are recorded year-round in
coastal Atlantic waters, which has further reinforced the con-
cept that a seasonal migration to warmer waters for calving
or breeding is not as predictable as that observed for other
baleen whales like the humpback whale (Megaptera
novaeangliae). Because the North Atlantic fin whale popula-
tion is thought to exceed 50 000 animals (Sigurjonsson,
1995) and fin whale calls are quite loud and are produced in
long series (183 dB re 1 lPa at 1 m; Cummings and Thomp-
son, 1994), these sounds are a significant contributor to
ocean ambient sound, seasonally raising sound levels in
some areas by as much as 25 dB (Curtis et al., 1999).
III. METHODS
From 1999 to 2008, hydroacoustic records of Atlantic
ocean-basin earthquakes were collected by an international
consortium of geophysicists studying the Mid-Atlantic Ridge
(Smith et al., 2002;Dziak et al., 2004;Sima˜o et al., 2010).
The instruments used in these experiments, autonomous
hydrophones designed for continuous deep-sea recording
(Fox et al., 2001), were developed by engineers from
NOAA’s Pacific Marine Environmental Laboratory (PMEL)
and Oregon State University. Each mooring package con-
sisted of an anchor, an acoustic release, a hydrophone and ti-
tanium pressure case containing a logging system, and a
syntactic foam float designed to suspend the hydrophone in
the Deep Sound Channel (a depth of 900 m) to maximize
acoustic coverage of the area. Arrays of these hydrophone
moorings monitored sound continuously, recording the am-
bient acoustic signal to disk at a sampling rate of 110 Hz,
250 Hz or 500 Hz. This low sample rate was designed to effi-
ciently monitor seismicity from earthquakes but was also
adequate for monitoring low-frequency sounds from seismic
airguns and some marine mammals, including fin whales.
These instruments were moored along the east and west
flanks of the Mid-Atlantic Ridge between 16N and 50N
in three basic arrays (Fig. 1). The location of the hydrophone
moorings changed somewhat each year depending on the
section of the ridge that was monitored, but arrays were con-
figured to straddle the ridge.
After the archived data were recovered, we used the bio-
acoustics software package Ishmael (Mellinger, 2001; see
http://www.bioacoustics.us/Ishmael.html) to detect airgun
sounds automatically. Airgun sounds are broadband, very
repetitive pulsive sounds [Fig. 2(a)]; therefore, we used a
combination of an energy sum detector (10–55 Hz) and a
sequence detector (Mellinger et al., 2004; autocorrelation
window length ¼200 s, hop size fraction ¼0.25, period
length ¼9–25 s) to configure the automatic detection algo-
rithm. Detections were then visually confirmed by an experi-
enced analyst. Because seismic surveys are typically
conducted over periods of weeks or months, we verified the
number of days in a month with airgun sounds.
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The high source level of airguns made it possible to
simultaneously record the same series of airgun sounds at
more than one site. To understand further the seasonal and
spatial patterns (or lack thereof) of airgun signals, we acous-
tically located the sources of airguns. Locating airgun sound
sources proved challenging because clear airgun signals
were often blocked by the bathymetry or absent due to gaps
in recording. Because of the large distances involved, and
consequent distortion of airgun signals, locations derived
acoustically from only three sites often had large errors, so
we located airgun sound sources only when signals were
received clearly at four or more sites. When the first or last
airgun shot in a series was clear at four or more sites, we
used a modified least-squares optimization method (Fox
et al., 2001) in IDL
V
R
(Interactive Data Language Research
Systems, Boulder, Co.) to locate the approximate source of
the airgun pulses. We also examined propagation of sounds
from airgun source locations to our hydrophones using the
acoustic propagation code RAM (Collins, 1993).
The acoustic record was also analyzed for fin whale
calls. Because the low sampling rate of the recorders limited
the available frequency band, we targeted the fin whale
20 Hz pulse [Fig. 2(b)]. We calculated long-term spectro-
grams and used methods similar to those developed by
Curtis et al. (1999) and 
Sirovic´et al. (2004) to derive an
index of fin whale calling. This fin index, a numeric value
gauging the daily average of normalized spectral energy
over the vocalization band of this species, was derived by
first calculating a log-scaled spectrogram S(t,f) of the sound
(spectrogram frame and FFT size ¼1 s, overlap ¼0.5 s,
FIG. 1. Locations of 12 autonomous hydrophone moorings (stars) moored
along the Mid-Atlantic Ridge, and approximate locations (dotted boxes with
circles) of seismic airgun activity located via the array.
FIG. 2. (Color online) (a) Spectrogram and time of series of airgun pulses recorded 19 August 2002 on the 26N50
W hydrophone (spectrogram parameters:
frame and FFT length 4.7 s (512 samples) overlap 0.75, Hamming window, for a filter bandwidth of 0.9 Hz). (b) Spectrogram and time series of fin whale
20 Hz pulses recorded 05 November 2005 on the 32N35
W hydrophone (spectrogram parameters: frame and FFT length 8.2 s (2048 samples) overlap 0.75,
Hamming window for a filter bandwidth of 0.50 Hz). (c) Example of co-occurrence of airgun and fin whale sounds. In instances where the fin whale sounds
were of lower amplitude than those in this figure, fin whale pulses would be obscured by airgun noise. (d) Example of airgun pulses (spectrogram parameters:
frame and FFT length 2.3 s (256 samples) overlap 0.75, Hamming window, for a filter bandwidth of 1.8 Hz) recorded over 3900km from the source. These
sounds were recorded on the 32N35
W hydrophone; the survey vessel producing them was acoustically located in Brazilian waters.
1104 J. Acoust. Soc. Am., Vol. 131, No. 2, February 2012 Nieukirk et al.: Atlantic airgun and fin whale sounds
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Hann window, for a filter bandwidth of 4.0 Hz). The spectro-
gram was then averaged over 1-day periods (Fig. 3). Next,
normalization was performed by finding, at each time step t,
the noise floor n
50
(t): the median (50th-percentile) value in
the spectrum at time tbetween the frequencies of 0 and
55 Hz. This was subtracted from the spectrogram at that time
step, with negative values converted to 0, to produce the nor-
malized spectrogram 
S(t,f):
^
Sðt;fÞ¼Sðt;fÞn50ðtÞ:
Finally, the fin index I(t) was calculated at each time step t
as the sum of the normalized spectrogram values between
the frequencies of f
0
¼19.0 and f
1
¼22.0 Hz, a band
designed to exclude sound from blue whales that are near in
frequency:
lðtÞ¼X
f1
f¼f0
maxð^
Sðt;fÞ;0Þ:
This fin index ranged from 0 to approximately 12 and
depicted a relative estimate of fin whale calling in the data.
To ensure background noise was not confounding the index,
we also plotted the levels of sound in the adjacent frequency
bands and compared these to the index curve; if overall trends
in the noise and fin index curves were similar the fin index
was not used. Fin whale index numbers were then checked by
an analyst who examined the raw spectrogram data, verified
the presence of fin whale calls, and confirmed that the
spectrogram-derived indices were indeed representative of the
actual level of fin whale calls. Least squares linear regression
was used to analyze seasonal and geographic trends in the
data and results were considered significant at the p¼0.05
level. We were not able to locate individual vocalizing fin
whales due to the wide spacing of the hydrophone moorings.
IV. RESULTS
During the ten years of this study, over 246 000 hours of
acoustic data were collected and analyzed. Some deploy-
ments experienced hard drive failures, and in a few instances
instruments and their data were not recovered because of
malfunctioning acoustic releases or other failures of the
mooring hardware. As mentioned, the hydrophone moorings
were also deployed at different locations at different times
throughout the study area. This resulted in incomplete (non-
continuous) 10-year acoustic coverage at each mooring loca-
tion, but the results were adequate for observing latitudinal
and seasonal trends in airgun sounds and fin whale calling.
In addition, the position of the moorings on either side of the
ridge may have resulted in bathymetric blocking of some of
the sounds of interest; we assume the results represent a min-
imum estimate of fin whale calling behavior and airgun use
in the mid-Atlantic.
A. Airguns
Sounds from seismic surveys were recorded frequently
at all sites and in all years of the study (Fig. 4). Recorded air-
gun sound levels fluctuated over time, but airguns were
recorded during at least 9 months each year at every site. In
many months, more than 80% of the days in the month con-
tained sounds from airguns. During 2003 and 2005, the per-
centage of days in a month with airgun sounds routinely
exceeded 95% at the southernmost sites.
To evaluate seasonal patterns in the data, we examined
the southern array, which had almost six years (1999–2005)
of nearly continuous acoustic coverage. In 1999–2001, we
recorded airgun sounds throughout the year; levels peaked
during the summer months, probably because airgun survey
ships are most active in the northern hemisphere during the
summer (1999–2001 data published in Nieukirk et al.,
2004). During 2001–2002, this trend continued. In 2003,
these high levels of airgun activity were also observed in
summer, but levels remained high (>80% days/month) into
the winter months. In the first few years of the study, there
were fewer days with airguns sounds in February through
April, but that pattern did not persist into the later years.
We examined the acoustic data collected during
2002–2003 to identify spatial patterns in airgun noise levels.
FIG. 3. (Color online) Example
calculation of the fin index: (a) Log-
scaled spectrogram S(t,f) of the
sound (spectrogram frame and FFT
size ¼1 s, overlap ¼0.5 s, Hann
window, for a filter bandwidth of
4.0 Hz) and then averaged over
1-day periods; (b) normalized spec-
trogram 
S(t,f) (at each time step t,
the noise floor n
50
(t) was subtracted
from the spectrogram at that time
step, with negative values converted
to 0); (c) resulting fin index, or rela-
tive estimate of fin whale calling; (d)
levels of sound in the adjacent fre-
quency bands (upper: 40–45 Hz;
lower: 8–13 Hz), used to ensure tran-
sient noise was not affecting the fin
whale index.
J. Acoust. Soc. Am., Vol. 131, No. 2, February 2012 Nieukirk et al.: Atlantic airgun and fin whale sounds 1105
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During this time period, seven instruments were deployed
with latitudinal coverage from approximately 16–50N.
Extensive airgun activity was observed on days in July–-
January at most sites. On the instruments at 26N, extensive
airgun activity (>80% of days in a month) also continued
into the winter months. There were no apparent east-west
trends or north-south trends in airgun noise.
Locating the sources of airgun sounds revealed that
three geographic areas were major sources of airgun signals:
Newfoundland, northeastern Brazil, and Senegal and Mauri-
tania in West Africa (Fig. 1). Extensive airgun activity was
observed off Newfoundland during boreal summer months,
and shifted to off Brazil and Africa during the winter
months. In some cases, airgun sounds were recorded almost
4000 km from survey vessels working off Brazil.
Acoustic propagation modeling from airgun source loca-
tions off the coast of northeast Brazil revealed large (>40 dB)
variability in propagation loss over distances of tens of kilo-
meters and depth ranges of several hundred meters. Since
these ranges are smaller than our uncertainty in the position of
calling fin whales, we were unable to correlate changes in fin
whale calling behavior with changes in received levels of
airguns.
B. Fin whales
The 20 Hz pulses produced by fin whales were recorded
at all sites in this experiment from approximately August to
April of each year. Our proxy for the relative levels of call-
ing fin whales, the fin index, peaked during late December to
early January for most sites and most years (Fig. 5). In all
cases the fin index did not appear to be confounded by noise
in the adjacent frequencies. Fin index levels were the highest
at the sites north of 32N. Although the fin index at the
FIG. 4. Seasonal patterns of airgun pulses detected in data at the 12 mooring sites. Black bars represent percentage of days/month that airgun pulses were
detected. Gray bars indicate periods for which there were no data available.
1106 J. Acoust. Soc. Am., Vol. 131, No. 2, February 2012 Nieukirk et al.: Atlantic airgun and fin whale sounds
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southernmost sites also peaked in December–January, maxi-
mum index levels were approximately half of those observed
at the more northerly sites. At some sites, fin index level
peaks were quite distinct, as levels increased gradually in the
late summer and fall, peaked in winter with clear maxima
(i.e., at 50N24
W and at 32N35
W), and then began
decreasing during the spring months. At other sites (e.g.,
42N26
W), levels increased in July, peaked at much
lower levels in August, then fluctuated during August–
December and again returned to minimum levels in spring. At
the 32N35
W site, an area for which we had data spanning
10 years, fin index levels increased almost 2 months later than
at the sites to the north; this pattern was consistent over multi-
ple years. There were no clear trends in the time of peak call-
ing with latitude (R
2
¼0.02, p ¼0.41). However, there was a
significant increase in the annual peak fin whale call index
during the experiment (R
2
¼0.47, p <0.001; Fig. 6).
C. Overlap in airgun and fin whale sounds
Because the airgun activity levels were high during
summer, fall and winter months, these high noise levels
clearly occur during times when fin whale calling activity is
frequent [Fig. 2(c)]. Fin whale seasonal calling patterns were
relatively consistent at each site, but airgun patterns varied
from year to year. Thus, in some years (e.g., 2005) there was
a great deal of overlap of peak fin whale calling activity and
high airgun levels, while in others (e.g., 2001) we recorded
lower levels of airgun activity during times when high levels
of fin whale 20 Hz pulses were recorded (Fig. 7).
FIG. 5. Seasonal patterns of fin
whale 20 Hz pulses detected in data
at the 12 mooring sites. Black dots
represent the calculated fin index, or
relative estimate of fin whale calling.
Gray bars indicate periods for which
there were no data available.
FIG. 6. Normalized annual fin index peak. For each site, data were normal-
ized by dividing the annual peak index by the maximum peak index over all
years. Linear regression trend line: R
2
¼0.47, p <0.001. The positive trend
indicates that fin whale calling increased over the duration of this study.
J. Acoust. Soc. Am., Vol. 131, No. 2, February 2012 Nieukirk et al.: Atlantic airgun and fin whale sounds 1107
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V. DISCUSSION
A. Airgun sounds
From a low-frequency ocean acoustic dataset, we were
able to conduct a long-term, basin-spanning observation of
airgun noise levels in an area that is rarely surveyed but is
likely the migratory corridor for numerous species of ceta-
ceans. Airgun sounds were recorded at all sites, in all months
of the year, and at distances far from the source. These dis-
tances should not be surprising, as pulses from a geophysical
survey off the California coast were recorded on land seis-
mometers 6100 km from the source (Okal and Talandier,
1986). Because our instruments were located in the deep
sound channel, seismic survey vessels working in both the
northern and southern hemispheres and the eastern and west-
ern Atlantic were recorded despite the remote mid-ocean
location of these hydrophone arrays. When multiple sources
of airguns were recorded simultaneously, the resulting high
levels of noise usually obscured any biological sounds in our
acoustic data (see also Clark and Charif, 1998;McDonald
et al., 1995). During such periods, the sound from airguns,
which is usually considered a transient noise (Richardson
et al., 1995;McDonald et al., 1995) actually becomes a
prevalent part of ambient/background noise levels for this
area. During a few recording time periods, we had good
acoustic coverage of an area but detected no airgun sounds;
this likely happened because survey ships were not operating
during this time period.
B. Fin whale calling
The focal species for this study, the fin whale, is a
highly vocal whale that produces loud, 20 Hz pulse vocaliza-
tions in long sequences that are ideal for analysis via long-
term spectral techniques. Our proxy for fin whale abundance,
the fin index, yielded consistent results and did not appear to
be affected by confounding noise in the 19–22 Hz band. Fin
index data also agreed with a previous analysis of the
1999–2001 data in which we used visual inspection of spec-
trograms to count presence/absence of fin whale pulses per
hour [Nieukirk et al., 2004; Fig. 3(a)]. The fin whale calling
patterns we observed are similar to more coastal studies of
Atlantic SOSUS data to the north (coasts of Britain and Ire-
land; Charif and Clark, 2009) and west (Clark and Gagnon,
2002;Watkins et al., 1987) of our study area. In our study,
fin whale vocalizations were detected from August to April,
with peaks November to January for most sites. Charif and
Clark (2009) reviewed ten years of SOSUS data collected
off the coasts of Britain and Ireland and report fin whale
20 Hz calls in all months of the year, with a peak in
FIG. 7. (Color online) Seasonal patterns of airguns (black bars, left y axis) and fin index (light gray line, right y axis) for sites in the southern array. Dark gray
bars indicate periods for which there were no data available.
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December and January. SOSUS data from the western Atlan-
tic revealed patterns similar to ours—calls in October
through June, rare detections during the summer months,
and peak calling later in the year, during March and April.
At some sites there were no clear peaks in calling levels, but
instead levels were variable for months (July–February),
possibly because we were recording many distant whales, or
because calling whales were constantly moving into and out
of the range of these instruments.
It is unclear why the fin index was higher at the more
northerly sites. Because a clear wintertime migration to
warmer southern waters has not been observed in fin whales,
the high calling levels we observed may simply be a reflec-
tion of the winter breeding display of high numbers of
whales [>25 000; North Atlantic Marine Mammal Commis-
sion (NAMMCO), 2004] estimated to be feeding in the
waters off Greenland and Iceland. The regional differences
observed in fin index levels may also be reflected in the
long-term spectrogram data. Animals from more northern
latitudes have a shorter inter-pulse interval than whales sing-
ing in more southern Atlantic waters (Watkins et al., 1987;
Hatch and Clark, 2004), a difference that may result in less
acoustic power in the 19–22 Hz frequency band and thus a
lower fin index for the southern sites; this could be modeled
in future studies. When data were available for multiple
years, index levels often increased over time (e.g., at 32N
35W), a pattern opposite of the trend observed by Charif
and Clark (2009) to the northeast of our study. Because of
our data collection methods, we are unable to determine if
these higher calling levels are from louder whales or more
vocalizations.
C. Airgun and fin whale sounds
What do the observed noise levels mean for an animal
using this mid-Atlantic migratory corridor? Loud, broadband
sounds like the pulses produced by seismic airguns have the
potential to adversely affect baleen whales, either directly by
impacting the animals physically (temporary or permanent
hearing threshold shift (deafness), physiological changes) or
indirectly by affecting their prey, changing their behavior
(startle response, moving away from source, changing vocal-
izations), or by masking their vocalizations (Richardson
et al., 1995;Gordon et al., 2004;Compton et al., 2008). In
past studies of baleen whales exposed to seismic airgun
sounds, results have been varied. Clark and Gagnon (2006)
observed that singing fin whales stopped singing when
exposed to airgun sounds from three or more vessels operat-
ing simultaneously, and stayed silent throughout the days of
the survey. Castellote et al. (2012) observed changes in fin
whale vocalizations as well as movement away from vessels
conducting seismic surveys. Di Iorio and Clark (2010) found
that blue whales called consistently more on days with sound
from a seismic exploration “sparker” than on days without it.
McDonald et al. (1995) observed that a blue whale stopped
vocalizing when within 10 km of an active seismic vessel.
Migrating bowhead whales (Balaena mysticetus) avoided
airguns at ranges exceeding 20 km and received levels of
120–130 dB re1 lPa RMS (Richardson et al., 1999), while
bowhead whales have been observed to change their surfac-
ing patterns at ranges up to 73 km from seismic survey ves-
sels (Gordon et al., 2004, Table II). There is some evidence
that the behavioral state of baleen whales (e.g., feeding or
migrating, Gordon et al., 2004; resting behavior, McCauley
et al., 1998) and the proximity to the noise source affect a
whale’s level of reaction to airgun sounds. Migrating whales
and those individuals exposed to received noise levels
exceeding 150 dB were observed to exhibit the strongest
reactions (Gordon et al., 2004, Table II). In this study, the
migratory fin whales we recorded are occasionally exposed
to seismic research vessels surveying areas of the Mid-
Atlantic Ridge in close proximity to our moorings (cf. Nieu-
kirk et al., 2004, Fig. 1), but the sources of the majority of
airgun signals are thousands of kilometers from the mid-
Atlantic. Individuals were thus far from the source of noise,
and direct effects from airguns were not likely.
The most likely effect of the observed frequent seismic
noise is a decrease in the effective range of communication
among whales using these waters. Masking of vocalizations
occurs when ambient noise levels make the signal of interest
less detectable and is of concern for a highly mobile species
like the fin whale that may be using low-frequency sounds to
communicate over long distances while migrating thousands
of kilometers (Tyack, 2008;Clark et al., 2009). Masking of
these 20 Hz vocalizations is possible because seismic pulses
are, like fin pulses, broadband, very low-frequency sounds
(Gordon et al., 2004) that repeat every 10–25 s (Hatch and
Clark, 2004). There is also clear geographic and seasonal
overlap in these two low-frequency sounds: fin whale sounds
were recorded at all sites, as were airgun sounds, and fin
whale vocalization levels increased during the late summer
and fall months, a time when airgun noise levels were often
high (>80% days/month with airgun sounds) at all sites. In
addition, because airgun pulses increase in duration as the
distance from the source increases, and because this noise is
also produced in long sequences for prolonged periods of
time by multiple sources around the Atlantic, masking of fin
whale vocalizations is quite possible, though masking is
eventually limited by the decrease in intensity with distance
from the airgun source. Some authors have argued that
sounds recorded via hydrophone arrays moored in the
SOFAR channel would not be heard by whales (McDonald
et al., 1995). We argue that because sound waves propagat-
ing along the deep sound channel axis are in fact being
refracted above and below it (Urick, 1983), sound from dis-
tant airguns does in fact reach shallower depths where
whales vocalize.
Although there are clear overlaps in the geographic and
seasonal occurrences of these two low-frequency sounds,
assessing the impact of this noise on fin whale communication
and the biological significance to individuals or populations is
difficult. We are unable to precisely locate a vocalizing whale
with this array because of the distance between instruments
(>700 km), but given that the source level of a fin whale call
is 183 dB re 1 lPa at 1 m (Cummings and Thompson,
1994), we estimate that the recorded fin whales are within
tens of kilometers of our instruments (see Stafford et al.,
2007). Although we can roughly estimate the location of the
J. Acoust. Soc. Am., Vol. 131, No. 2, February 2012 Nieukirk et al.: Atlantic airgun and fin whale sounds 1109
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signaling/vocalizing whale, we do not know the distance over
which fin whales communicate (Tyack, 2008). If the receiver
of the signal is close to the vocalizing whale, and both animals
are far from the source of airgun pulses, then masking may be
less likely. Evidence from field studies suggests fin whales
(Watkins and Schevill, 1979) and humpback whales (Tyack
and Whitehead, 1983) are communicating over at least tens of
kilometers, while Payne and Webb (1971) suggested whales
could use low-frequency sound to communicate over thou-
sands of miles. More recent models estimate fin whale calls
could propagate over 400 km (Spiesberger and Fristrup,
1990). Di Iorio and Clark (2010) point out that for “animals
engaged in long-term singing directed to a distant audience,
information loss is minor if singing is temporarily inter-
rupted.” However, if animals stop signaling for long periods
of time or avoid or abandon habitat, there could be significant
population-level effects, especially for endangered species
(Tyack, 2008). Current evidence suggests that the 20 Hz pulse
vocalization is produced by males (Watkins et al., 1987)and
is likely a breeding display to attract females, perhaps to
patchily distributed food (Croll et al., 2002). The contracted
range of fin whale populations in post-whaling years may
have increased the separation of whales during the breeding
season, and a decrease in communication range could
adversely affect recovery of this endangered species (NRC,
2003;Tyack, 2008).
Like other vocal animals, whales can compensate for
increased ambient noise levels and avoid masking by vocal-
izing more often, changing the timing or frequency of vocal-
izations, or increasing the source level of the sounds they
produce. Right whales have been observed to increase the
frequency of their calls in noisy areas (Parks et al., 2007),
blue whales increased their rate of calling in response to air-
gun sounds (Di Iorio and Clark, 2010), humpback whales
increased the length of songs in response to sonar noise
(Miller et al., 2000), and killer whales increased the ampli-
tude of their vocalizations in response to increased levels of
background noise (Holt et al., 2009). The biological costs
associated with such changes in signaling are unclear. For
the fin whale, the repetitive nature of the long series of 20 Hz
pulses increases the chance that these short sounds could be
heard in a noisy environment. Further scrutiny of fin whale
vocalization patterns and source levels during times of high
and low seismic noise levels may reveal the extent to which
these animals may be compensating for increased levels of
anthropogenic sound. Future studies with a tighter array ge-
ometry that allows localization of calling fin whales will be
necessary to answer such questions.
VI. CONCLUSIONS
In this study, our goal was to document the levels of air-
gun noise and fin whale sounds that were recorded in this
mid-Atlantic long-term, hydroacoustic data set. Despite the
remote location of this array, significant levels of biological
and anthropogenic sounds were recorded during the ten years
of this study. Seismic airgun noise overlapped fin whale calls
geographically and seasonally in our acoustic records. In
some locations, airgun noise occurred quite frequently
(>80% days/month) for more than 12 consecutive months.
Most of the seismic survey vessels we located were operat-
ing in areas that are important, if not critical, to many endan-
gered marine mammal species. These areas included waters
northeast of Brazil, west of North Africa, and south of New-
foundland. Of particular concern is the seismic noise origi-
nating in the waters off Newfoundland, an area of vital
importance to the critically endangered northern right whale
(Eubalaena glacialis). Because of the efficient propagation
of this loud, low-frequency noise, whales are likely exposed
to these sounds not only on their feeding grounds but also
during migration. Given our growing understanding of the
impacts of ocean noise on sensitive marine mammals, we
argue that the cumulative impacts of this noise and other
anthropogenic sounds should be carefully considered, espe-
cially in light of other potential stressors such as climate
change and pollution/contaminant loads. This basin-wide
study has reinforced the fact that ocean noise in an interna-
tional problem and should be managed as such.
ACKNOWLEDGMENTS
Funding for the deployment, maintenance, and data
analysis for the hydrophone array along the Mid-Atlantic
Ridge was provided by National Science Foundation Grants
to R.P.D. No. OCE-9811575, No. OCE-0137164, and No.
OCE-0201692, and by the Institut National des Sciences
de l’Univers (a department of the Centre National de la re-
cherche Scientifique, France). Analysis and writing were
supported by Office of Naval Research Grants No. N00014-
00-F-0395 and No. N00014-03-1-0099, Naval Postgraduate
School Grants No. N00244-10-1-0047 and No. N00244-11-
1-0026, and NOAA NMFS Contract No. AB133F08SE4903
to D.K.M. Development of the hydrophone instruments was
supported by in-kind funding from NOAA PMEL. We are
indebted to Haru Matsumoto for design and construction of
the instruments used in this experiment; Andy Lau for the
development of some of the software used for data analysis;
Matt Fowler and Joe Haxel for instrument deployment and
recovery, and Sarah Follett, Rita Bento, and Andra Bobbitt
for their help with various stages of the analysis. Lastly,
SLN would like to thank Chris Fox for encouraging me to
investigate airgun noise in our acoustic datasets. This is
PMEL contribution #3515.
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