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Target Strength of Southern Resident Killer Whales (Orcinus orca):
Measurement and Modeling
Jinshan Xua, Z. Daniel Denga,*, Thomas J. Carlsona, Brian Mooreb
a
Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA
b
*
BioSonics, Inc., 4027 Leary Way NW, Seattle, WA 98107, USA
Corresponding author at: Tel.: +1 509 372-6120; fax: +1 509 372 6089; E-mail address:
zhiqun.deng@pnl.gov
Abstract— A major criterion for permitting the deployment of tidal turbines in Washington State’s
Puget Sound is management of risk of injury to killer whales from collision with moving turbine blades.
An active monitoring system is being proposed to detect and track killer whales within proximity of
turbines and alert turbine operators to their presence and location to permit temporary turbine shutdown
when the risk of collision is high. Knowledge of the target strength of killer whales is critical to the
design and application of active acoustic monitoring systems. In 1996 a study of the target strength
directivity of a 2.2 m long bottlenose dolphin at an insonifying frequency of 67 kHz was performed.
Noting that killer whales, which are dolphins, are morphologically similar to bottlenose dolphins and
then assuming allometry, we estimated the relative broadside and tail aspect TS of a 7.5 m long adult
killer whale at an insonifying frequency of 67 kHz to be −8dB and −28 dB respectively. We used a
three-layer model for plane wave reflection of sound at 200 kHz from the lung of killer whales to
estimate their target strength. We assessed the accuracy of our killer whale target strength estimates by
comparing them with target strength estimates of free swimming killer whales obtained using a splitbeam active acoustic system operating at 200 kHz. The killer whale target strength estimates based on
the preliminary model were in good agreement with those obtained for free swimming killer whales.
Keywords - target strength; Southern Resident killer whales; active sonar system; tidal power
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I.
INTRODUCTION
Prototype marine hydrokinetic energy devices are in the process of being deployed in coastal waters to
evaluate their performance and any environmental consequences of operation. No tidal power generating
devices are currently deployed in the United States. A Federal Energy Regulatory Commission
preliminary permit has been granted to Snohomish County Public Utility District No. 1 to deploy two
tidal turbines built by OpenHydro (Dublin, Ireland) at a site in Admiralty Inlet in Washington’s Puget
Sound to study their operation over a 5-year period. A criterion for final approval for deployment of tidal
turbines by regulatory authorities is demonstration of capability to manage the risk of injury to killer
whales from strike by moving turbine blades. Pacific Northwest National Laboratory researchers are
designing and testing a prototype of a marine mammal alert system (MMAS) for detecting the presence
of killer whales in the immediate vicinity of tidal turbines. The system will inform tidal turbine operators
about the presence of the killer whales to facilitate shut down of the turbines when there is high risk of
collision of the whale with the operating turbine. Both passive and active monitoring systems are being
considered for the MMAS. The passive monitoring system is being developed by modifying an energybased juvenile salmon acoustic telemetry system (McMichael et al., 2010; Deng et al. 2011; Weiland et
al., 2011). Several commercially available sonars are being evaluated for the active monitoring system.
An object in water with acoustic impedance different from water scatters a portion of acoustic energy
incident from an active acoustic source back toward the insonifying acoustic system or other receivers
within range. The ratio of energy incident on the object to that backscattered from the object is defined as
the target strength (TS) for the object. With the exception of a sphere of uniform composition, the target
strength of an object, such as a fish or whale, is a complex function of the shape of the object and its
orientation to the insonifying sonar system. Target strength is a parameter in the sonar equation and is
required for the design of a sonar system to optimize design for detection of the targets of interest. The
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TS of an object is not single valued but a complex function of geometry, size, composition, and frequency
of the insonifying sound. The TS of simple geometric shapes with acoustic impedance significantly
different from water can be accurately derived analytically. The TS of more complicated shapes with
complex composition are typically experimentally determined. However, when measurement of TS is not
feasible because of cost or technical constraints, such as the case with killer whales, a combination of
analytical analysis and numerical simulation with consideration of the composition of complex objects
can be used to estimate TS.
Experimental determination of the TS of animals such as fish is typically done in a laboratory where
the animal or acoustic measurement system can be manipulated to insonify the whole animal or parts of
the animal over a range of aspects (Love, 1971; Foote, 1980; Clay, 1991; Reeder and Stanton, 2004;
Reeder et al., 2004). For large animals, such as whales, very few TS measurements have been reported
due to the technical difficulty of performing TS measurements on unconstrained animals in the open
ocean. The first measurement of a sperm whale was made by Dunn (1969) using an explosive charge as
the sound source and a calibrated sonobuoy as the receiver. Dunn reported a TS of −8 dB at 1 kHz for an
adult sperm whale. Levenson (1974) reported a similar bistatic maximum TS of 14.4 dB at a center
frequency of 12 kHz for sperm whales. Love (1971) measured the TS of humpback whales at a range of
about 70 m. Love reported the TS of a 14 m adult humpback insonified at 20 kHz to be 7 dB near side
aspect and −4 dB near head side aspect. The TS of a 9 m long juvenile humpback was reported to have a
TS of 2 dB near broadside aspect at 10 kHz (Love, 1971). Miller and Potter (2001) reported two TS
measurements made at sea for humpback and northern right whales with an 86.25-kHz continuous-wave
3-ms duration pulse transmitted by a phased-array sonar. The broadside TS of an adult humpback whale
was observed to be 4 dB. Miller and Potter observed the broadside TS of a 15 m long right whale to be
between −4 and −8 dB; they also reported that the head aspect TS of 8 m long juvenile right whales
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ranges between −7 to −13 dB. Most recently, Lucifredi and Stein (2007) reported an aspect-dependent
gray whale TS measurement taken at about 500-m range using a vertical line array sonar frequency
centered at 23 kHz. The reported TS measurements for adult gray whales ranged from −2.9 dB at tail
aspect to 12.8 dB at broadside aspect. An active sonar system was used to study killer whale hunting
behavior in Norwegian waters (Simila, 1997; Nottestad and Axelson, 1999; Nottestad and Simila, 2001;
Nottestad et al., 2002a, 2002b). A comprehensive and relatively well-controlled experiment was
conducted to insonify a tagged killer whale using two commercial active sonar systems (the Simrad
Model SP90 (20-30 kHz) and Simrad Model SH80 (110 to 120 kHz), Kongsberg Maritime AS, Horten,
Norway). The goal of the study was to evaluate the feasibility of using active acoustic systems to detect
killer whales and observe their behavior in the Lofoten fjords of Northern Norway (Kvadsheim et al.,
2007). However, TS observations of killer whales, if obtained during this study, were not reported. Au
(1996) measured the TS directivity at 67 kHz of a well-trained bottlenose dolphin in a pool like facility.
He also insonified parts of the dolphin to identify the major contributors to whole body TS. Au observed
relatively low TS for most portions of the dolphin's body with exception of strong echoes from the lungs.
Au’s observations indicate that the air filled lung of whales, and presumably other marine mammals, is a
major reflector of sound, which is similar to findings for fish that indicate that the swim bladder is the
major reflector of sound in the species of fish that have one (Clay and Horne, 1994).
The target strength measurements for large marine mammal at sea are very important but difficult to
obtain. Usually, when these measurements are collected, they are most likely only for a specific animal
and certain frequency and there are a lot of gaps that we need to fill before we can apply these
measurements to practical applications. In this paper, we report the preliminary results of TS modeling
we performed to extrapolate the TS measurements of bottlenose dolphin provided by Au (1996) to killer
whales. We also report the results of analysis of echo data we acquired for three freely swimming killer
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whales in 2008 using a 200 kHz split-beam sonar and additional analysis we conducted to explore
differences between the in-field TS measurement of killer whales and those we estimated by
extrapolation from Au’s data using a 3 layer acoustic scattering model (Miller and Potter 2001). This
model assumes that the lung is the major sound reflector in whales and estimates the plane wave
backscattering coefficient to account for the acoustic impedance and absorption of whales flesh and
blubber at different frequencies. Although it is simple and preliminary, it provides an instructive method
for comparing the available measurement of various species at different frequencies and useful
information for practical applications in the future.
II.
A.
MATERIALS AND METHODS
Measurements
In 2008, a DT-X Echosounder (BioSonics, Inc., Seattle, Washington) with a deck unit (BioSonics,
Inc.) was deployed to continuously monitor for killer whales at Lime Kiln Point on San Juan Island,
Washington State. BioSonics DT-X is a 9˚ (horizontal)× 6˚ (vertical) 200 kHz digital split-beam
echosounder, which has been developed and commercially available for the direct measurement of target
strength (Carlson and Jackson 1980). The split-beam echosounder has a transducer, which is usually
symmetrically divided into four quadrants (Ehrenberg 1974). This BioSonics DT-X echosounder has one
large element whose signal is used to determine the echo amplitude, while three smaller elements are
used to measure the off-axis angles. The primary goal was to demonstrate the capabilities of autonomous
scientific hydroacoustics systems by using an active sonar system to study distribution, abundance, and
behavior of killer whales moving past Lime Kiln Point.
The first field trial of the monitoring system was carried out from the R/V Gato Verde, a catamaran
sailing vessel with better stability than a monohull boat, at a depth of 14 m. The system transducer was
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mounted on a customized aluminum frame deployed at a depth of 1 m. The coordinates for the location of
the transducer were 48° 30.920′ N and 123° 9.168′ W. The transducer was aimed horizontally at a bearing
of 238° clockwise from magnetic north. The transducer was connected to the deck unit by power and
digital signal cables. The data collected during this field trial are discussed below.
The BioSonics DT-X system could be operated in the range from 5 to 250 m at 200 kHz. The system
transmitted a rectangular pulse 0.4 ms in duration at a rate of 2 pulses/second. On May 22, 2008, three
killer whales (two adults and one juvenile) were visually observed passing by the drifting R/V Gato
Verde. The fixed-aim sonar system detected the whales and recorded their approach as shown in the
echogram presented in Figure 1. The three rectangular frames (red dashed lines) in the echogram outline
three time segments containing sequences of echoes from the visually observed whales. The killer
whales were within the beam of the sonar system for about 3 minutes.
B.
Data Analysis
1)
Whale Behavior
The killer whale echo sequences were processed to identify the strongest echo returns. The strongest
echo returns in each segment that appeared to be from an individual whale were used to estimate the
individual whale’s TS. The phase information from the split-beam system for each echo return, when
combined with the aiming angles of the system’s transducer, permitted the azimuth and elevation angles
to the whales relative to the acoustic axis of the transducer to be estimated. The ranges to the targets were
also provided in the system output.
While the echoes from individual whales were easily discerned in the echo sequences, the visual
observations of the whales were not sufficiently detailed to unambiguously assign a particular whale to
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each echo sequence or to the echoes selected for estimation of TS. Therefore the target strength estimates
discussed below are not specific to particular whales.
2)
Target Strength
The echo returns from the whales were recorded in the BioSonics .dt4 format (Biosonics, 2008). A
quadrature amplitude demodulation technique (Proakis, 2000) was applied to the filtered analog echo
signals out of the system transducer to provide a digital record of the echo returns with a sampling
frequency of 41.667 kHz. The target strength was estimated by compensating the beam pattern in the
relevant direction relative to the on-axis direction. The detection threshold was set at −70 dB re 1μPa to
allow the target to be detected only in the direction on the beam axis.
3)
Aspect Direction/Incident Angle
The split-beam system processed the echo returns to estimate the azimuth and elevation of received
echoes relative to the transducer’s aiming direction. The angle of incidence of the transmitted pulse on a
whale was estimated by computing the angle of travel of the whale relative to the acoustic axis of the
system transducer from changes in range to the whale on successive echo returns. When range was
increasing the whale was assumed to be near tail aspect and when range was decreasing the whale was
assumed to be near head aspect. Similar methods have been used by others, specifically Levenson (1974)
and Lucifredi (2007).
4)
Target Strength Modeling of Killer Whales at 67 kHz
Assuming allometry in lung size between species of dolphins we used the TS directivity data of Au
(1996) for a bottlenose dolphin to estimate the aspect dependent TS for killer whales. In Au’s study, a
female bottlenose dolphin weighing 126 kg and measuring 2.2 m in length was trained to remain
stationary at a ―bite‖ station while it was insonified to determine its TS. The transducers of the
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measurement system were manipulated to obtain the TS directivity (i.e., aspect dependent TS) of the
dolphin. Three different signals were used to measure the animal’s TS (two frequency-modulated pulses
and a broadband click with a peak frequency of 67 kHz). The target strength estimates at each aspect
were averages from echoes of 20 pings acquired for each angle of insonification of the whole dolphin or
of specific portions of the dolphin (Au, 1996). The ratio of the lung length to the full body length of this
dolphin was about 0.286. Assuming that dolphins and killer whales share a similar morphological
structure and that lung size is proportional, a 7.5 m long killer whale (mature female) would have a lung
length of about 2.143 m. Given the difference in TS of a 0.6-m-diameter sphere and 2.143-m-diameter
sphere is about 10.77 dB (Urick, 1983) we estimate that a 7.5 m long killer whale will have a TS at 67
kHz about 10.77 dB higher than that for a 2.2 m long bottlenose dolphin.
5)
Three-Layer Model To Extrapolate Target Strength Modeling of Killer Whales at 200 kHz
Miller and Potter (2001) suggested the difference in TS estimates between gray and right whales was
due to the difference in the thickness of their blubber layers. They also suggested an approach to estimate
TS at other frequencies. Their method requires estimates of blubber thickness, its density and the speed
and attenuation of sound through the blubber.
The mathematic formula for three-layer reflection coefficients can be found in Medwin and Clay
(1997), Miller and Potter (2001), and Brekhovskikh and Lysanov (2003). The TS of killer whales at
higher frequencies such as 200 kHz can be estimated based on the plane wave reflection coefficient as a
function of frequency for the elements of a three-layer model (skin, blubber layer, and lung) for a whale
(Miller and Potter, 2001). Miller and Potter (2001) estimated the reflection coefficient for different
thickness of blubber using a compressional sound speed of 1700 m/s, density of blubber of 1200 kg/m3,
and attenuation of sound through blubber of 1 neper/m. In contrast, Jaffe et al. (2007) measured the
attenuation of 100 kHz sound through sirenia (manatee) blubber to be about 5.57 dB/(10MHzmm),
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which equals 6.64 neper/m. For our analysis we chose to use values of of 4.45 neper/m at 67 kHz, 1.53
neper/m at 23 kHz, and 13.28 neper/m at 200 kHz.
The plane wave reflection coefficients for a three-layer model of water–blubber–lung at three different
frequencies are shown in Figure 2 for an angle of incidence of 90 normal to the surface of the blubber. It
also shows the variability in the reflection coefficient as a function of blubber thickness, which increases
with frequency. Depending on the thickness of blubber layer, the difference in reflection coefficient
between the frequencies of 67 kHz and 200 kHz could vary from 4 dB up to 15 dB. These reflection
coefficient values can be used to estimate the TS of killer whales at 200 kHz.
III.
A.
RESULTS
Whale Behavior
Figure 3 shows the averaged distance, depth, and swimming speed estimates for the killer whales
observed a Lime Kiln Point. There were some unrealistic values for whale depth (above surface),
possibly due uncertainty in the tilt angle of the split beam transducer in the horizontal direction or
propagation effect such as multipath off the ocean surface of the transmitter pulse and/or echo. Other than
these observations, the trends in whale depth appear to be realistic given the visual observations of the
whales. The whales were initially detected by the sonar at a range of about 100 m, swimming at a depth
of about 2 or 3 m. They approached to within about 30 m from the sonar transducer at a depth of about 5
m. At this range from the transducer they turned and swam away at a deeper depth. The killer whales
were also observed to slow on their approach to the transducer then speed up after they turned and were
swimming away from the transducer.
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B.
Target Strength
Temperature and salinity were also measured by the BioSonics DT-X system and used to improve the
accuracy of sound speed estimation. The resulting estimated speed of sound used in the echo ranging
calculations was 1475.68 m/s. Killer whale TS was estimated using the echo sounder source level and the
two-way transmission loss for 200 kHz sound in sea water. To improve the TS estimation, the relative
angles between the sending acoustic beam axis and returning echo beam axis were used to compensate
for the narrow beam effect by computing the composite beam pattern for the transmitting beam and the
angle of arrival of the echo.
During the 3-min the whales were observed by the sonar system (Figure 3), the whales initially
approached the transducer at a fairly high swimming speed for about 20 s and then slowed and
approached closer to the transducer over a 45 s period. In the final 20 s of observation, the whales turned
from the transducer and swam away at higher speed until no longer detected by the sonar system. In each
period (as annotated in Figure 1 with red color frames), the incident angles should have different
statistics, because the aspect of insonification of the whales would have differed. During approach the
whales would have been insonified at near head aspect; during their turn at a range of aspects from near
head to broadside and finally near tail, and when swimming away at near tail aspect. For this reason, we
analyzed the data in these three sections separately.
Figures 4 illustrates the TS as a function as distance and time and provides histograms of TS data for
each. In the first section, the averaged TS was −33 dB and the averaged speed was about 2.1 m/s. In the
second section, the averaged TS was about −31 dB with an averaged speed of 1.6 m/s; it had highest TS
measure of −4 dB. In the third section, the averaged TS was −23 dB with an averaged speed of 2.7 m/s.
The three sections of observations where the whales were behaving differently and the aspect of the
whale to the sonar system differed had different TS statistics. The first one looked like a central-weighted
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Gaussian distribution. The second one had a long tail at higher TS values. The TS in the third section has
a bimodal distribution. The differences in the TS statistics for each segment are most likely due to the
differences in the aspects at which the whales were insonified and the complexity of swimming whales as
acoustic targets. The first segment had more TS measured at head aspect because the whales were
approaching the transducer. In the second segment the whales presented more of a broadside aspect to the
sonar system initially followed by more tail aspect. In the third segment the whales were at more of a tail
aspect.
In the next section we will consider in more detail the observed whale TS as a function of aspect to the
insonifying acoustic beam.
C.
Target Strength As Function of Aspect Direction/Incident Angle
Figure 5 shows histograms of the estimated TS and angle of incidence of the insonifying beams on the
whales for the three observation segments. In each set of histograms the frequency of occurrence of
observed TS values are in the upper panel and histograms for the frequency of occurrence of estimated
insonifying aspects are given in the lower panels.
In the first set of histograms (Figure 5a-b) that correspond to the first observation segment where the
whales were approaching the sonar transducer, most of the incident angles (lower panel) are less than
100° (0 is head aspect, 90 is broadside aspect, 180 is tail aspect); there are few observations above
100°. In the second set of histograms corresponding to the second observation segment (Figure 5c-d), the
lower panel shows the incident angles spread across the entire range from head to tail aspect but with
higher frequency of occurrence at broadside aspect (90). The third set of histograms for the third
observation segment when the whales were moving away from the sonar transducer (Figure 5e-f), the
incident angles are more frequent from broadside toward tail aspect. These estimates of the aspect of
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insonifying beam incidence are consistent with our interpretation of whale motion based on analysis of
the change in range with time of echoes from the whales.
D.
Target Strength As Function of Aspect Angle
The TS observations of three individual of killer whales as a function of the angle of incidence of the
acoustic system beam are shown in the polar plots of Figure 6 for the three observation segments. In the
first observation segment, most of the TS measurements are for head-on aspect because the whales were
approaching the transducer. The TS observations for this segment vary from about −45 dB at an
insonifying incident angle of 20° (near head) to −22 dB at an aspect angle of 80° (near broadside aspect)
(Figure 6a). In the second observation segment (Figure 6b), the highest TS observations, about −10 dB,
were at 90° incidence (broadside); however, there are also a few observations of high TS at 0° (head
aspect) and 180° (tail aspect). In the third observation segment (Figure 6c), the TS measurements were
primarily from incident angles larger than 90°, more toward tail aspect, as expected for whales that were
moving away from the sonar system. The highest TS value observed was −7 dB at broadside aspect. The
data also show a lobe in the directivity at about 110°, with TS values up to −13 dB. The lowest TS values
observed were near −40 dB.
E.
Target Strength Modeling of Killer Whales
The TS directivity estimates at 67 kHz for killer whales along with the TS directivity for bottlenose
dolphins measured by Au (1996) are shown in Figure 7. The estimated TS of killer whales is modeled on
the measurements of bottlenose dolphin reported by Au (1996) with a 10.77 dB offset to account for the
difference in the size of the lungs in the two dolphin species. The TS of an adult killer whale 7.5 m in
length at a frequency of 67 kHz is approximately −8 dB at the broadside aspect and −28 dB from the tailon aspect. This estimate is applicable for 7.5-m killer whales swimming close to the surface; however, the
TS of aquatic mammals vary with water depth because of changes in lung volume with increasing
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pressure. The change in TS with depth could be estimated assuming that lung volume would decrease by
50% for each doubling of hydrostatic pressure (Ridgeway and Howard, 1979).
IV.
DISCUSSION AND CONCLUSIONS
Field measurement of the TS of marine mammals in general and large whales in particular is very
difficult for many reasons. Data from the May 2008 deployment of a BioSonics 200 kHz active sonar
system at Lime Kiln Point provided a unique opportunity to estimate the TS of killer whales. The
analyses of these data provided estimates of killer whale TS over insonifying aspects from tail to head.
The TS measurements obtained at 200 kHz of killer whale TS can be compared with TS estimates
obtained using a preliminary three-layer model of acoustic reflectivity at 67 kHz.
The Southern Resident killer whale preys on adult Chinook salmon in Puget Sound and elsewhere in
the Pacific Ocean near the northwestern Pacific coast. Killer whales should develop a blubber layer but
not as thick as that of right whales or even humpbacks. In terms of overall size, killer whales are
relatively smaller than right whales (10.7 to 16.8 m long) and humpbacks (12.2 to 15.2 m long). Thus,
assuming killer whale blubber thickness is about 0.1 m, approximately −10 dB must be added to the TS
model of killer whales TS at 67 kHz. This adjustment to the 67 kHz estimate of TS directivity for killer
whales would result in 200 kHz TS values ranging between −18 dB to −38 dB depending upon aspect
angle. The TS estimates obtained so far are for a 7.5 m killer whale. However, the adult male killer
whales can be as much as 10 m in length. Given our proportionality arguments we would expect the TS
of larger killer whales to be as much as 3 dB higher than those for the smaller 7.5 m long whale. This
would increase the range of TS for the larger killer whales to −15 dB to −35 dB. The measurement data
for the free swimming killer whales at Lime Kiln from the first observation segment range from −22 dB
to −45 dB where the whales were believed to be observed at near head on aspect. Measurement data from
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the second observation segment yielded TS values ranging from −10 dB to −48 dB over a range of aspect
from head through broadside to tail. In observation segment three, TS observations ranged from −7 dB to
−40 dB for retreating whales insonified at the tail aspect. These TS measurements are relatively
comparable to the model for killer whales at 200 kHz, which predicts TS values within the range from
−15 dB to −35 dB. To our knowledge, no other field measurements of killer whales TS have been
reported in the literature.
The target strength model of killer whale TS is based on TS measurement of a bottlenose dolphin at 67
kHz and a three-layer reflection model used to extrapolate modeled killer whales TS values from 67 kHz
to 200 kHz. The TS of an adult killer whale was modeled for an insonifying frequency of 67 kHz by
assuming that a killer whale shares similar morphological structure with a bottlenose dolphin and that the
ratio of lung length to total length is the same across dolphin species and sizes of animals within species.
The TS of killer whales at higher frequencies - 200 kHz were modeled using a simplified three-layer
model with correction based on the estimated differences in acoustic reflection coefficients as a function
of blubber thickness at different frequencies. Interestingly, the averaged measurement data of three
different Lime Kiln observation segments show that the TS of killer whales are in the range of −11 dB
(broadside) to −43 dB (tail aspect). The modeling result gives range from −18 dB (broadside) to −38 dB
(tail aspect) at 200 kHz frequency. Both the modeling and measurements TS results are surprisingly low.
In Au’s 1996 measurement of the bottlenose dolphin, he found an unexpected decrease in TS at the
broadside aspect as the frequency increased from 23 to 45 kHz. Au explained the TS decrease might be
due to the anechoic properties of dolphin skin or blubber layer because, based on the formula provided in
Urick (1983), the TS for most underwater objects increases or remains constant with frequency.
However, the data used by Urick (1983) are mostly for simple geometric shapes of uniform composition
and are assumed to have a large impedance difference from water. None of these assumptions hold for the
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body of whales, which are complex, composed of layers with very different physical and acoustic
properties. Even when the three-layer model is simplified by not considering the skin and muscle, it
might still provide a better estimate of acoustic reflectance than the two-layer model with large
impedance mismatch used by Urick. These considerations might explain the unexpectedly low TS
measurements Au’s 1996 obtained for bottlenose dolphin and might also help explain the relatively low
killer whale at 200-kHz frequency.
As with any other experiment, unavoidable measurement errors are always a factor affecting
estimates. For example, the pulse length of the Biosonics DT-X system is 400 μs (user selectable from
0.1 ms to 1.0 ms), which is likely too short to insonify an entire whale; this is most certainly the case
when the whale is approaching or swimming away from the sonar. In that case, the TS of killer whale at
tail or head aspects would be underestimated. The TS of an underwater object is likely to be less at short
ranges than at long ranges. One reason is that a very directional active sonar signal fails to insonify the
entire target. Another reason is the target reflection as echo usually acts differently from point source,
depending on the size and shape of the underwater object.
The trajectory of whale motion in Figure 1 and the whales’ speed as a function of time in Figure 3
show that during the 3-min sonar observation period, the whales first were approaching the transducer for
about 20 s and then moved closest to the transducer for about 45 s. In the final 20 s, the whales moved
away with increased speed. Similar behavior was observed also in studies by both Love (1971) and
Lucifredi and Stein (2007) with center frequencies of 20 kHz and 23 kHz for humpback whales and gray
whales. They observed that the whales approached the transmit site, slowed down or even stopped to
listen for a while, and then moved on with increased speed. It is generally considered safe to use
echosounders at frequencies above 180 kHz in the vicinity of marine mammals because this is out of the
hearing range of most marine mammals. However, the recent study of a signal leakage of active sonar
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systems (Deng et al., 2010) shows that a signal other than the primary signal at lower frequency could
potentially fall into the hearing range of marine mammals (Southall et al., 2007), which is consistent with
the observation in this case that the killer whales were capable of hearing the 200 kHz sonar because of
transmit signal spectral components at frequencies within the hearing range of killer whales.
Acknowledgment
The work described in this article was funded by The Wind and Water Power Program of the U.S.
Department of Energy Office of Energy Efficiency and Renewable Energy. The study was conducted at
Pacific Northwest National Laboratory (PNNL) in Richland, Washington, which is operated by Battelle
for the U.S. Department of Energy. The authors thank Tim Acker of BioSonics Inc. for providing the
original data. We also thank Charlie Brandt, Andrea Copping, Jayson Martinez, Graysen Squeochs, and
Andrea Currie of PNNL, who provided comments and technical help in preparing the manuscript.
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List of Figures
FIGURE 1
Echogram acquired with a BioSonics split-beam DT-X system on May 22, 2008 at Lime Kiln State Park
that shows three time periods containing echoes from killer whales.
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FIGURE 2
Plane wave reflection coefficient as a function of blubber thickness for a three-layer model of water–
blubber–lung at three frequencies: 23-kHz, 67-kHz, and 200-kHz.
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FIGURE 3
The averaged distance from the sonar system transducer (a), swimming depth (b), and swimming speed
(c) of three killer whales.
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FIGURE 4
Target strengh estimates for three killer whales. The panels in the left column (a: Section 1; c: Section 2;
e: Section 3) show target strength observations for a killer whale as a function of distance from the sonar
system transducer and time. The panels in the right column (b: Section 1;d: Section 2; f: Section 3) show
a histogram of the whale target strength estimates.
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FIGURE 5
Histogram of the target strength (upper pannels) and acoustic beam incident angles (lower row panels)
for the three observational segments when whales were observed approaching (5a-b), turning away from
(5c-d), and retreating from the acoustic system transducer (5e-f).
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FIGURE 6
Target strength of three killer whales as function of acoustic beam incident aspect angles. (a)
Observation segment 1; (b) Obervation segment 2 ; (c) Observation segment 3.
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FIGURE 7
Polar plot of the estimated target strength of killer whale at frequency of 67 kHz. The red curve is the
target strength of bottle-nose dolphin measured by Au (1996).
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