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Potential for the use of Remotely Operated Vehicles (ROVs) as a platform for
passive acoustics.
Rodney Rountree1,Francis Juanes2,and Joseph E. Blue3
1Program Manager,Mount Hope Bay Natural Laboratory,School for Marine Science and Technology,
UMASS Dartmouth, 706 Rodney French Blvd., New Bedford, MA 02744-1221 rrountree@UMassD.Edu
2Department of Natural Resources, University of Massachusetts, Amherst, MA 01003
3President, Leviathan Legacy, Inc., 3313 Northglen Drive,Orlando, FL 32806 jblue46498@aol.com
Introduction
We are still largely ignorant of the distribution and behavior of the great majority of marine fish.
Possibly one of the greatest challenges to researchers attempting to study the behavioral ecology
of fishes is that of finding the fish in the first place. Since some fish are soniferous, acoustic detection
and tracking may offer methods of population assessment for management decisions. Passive
acoustic techniques can be a valuable tool for the identification of essential fish habitats (EFH) for
soniferous species. These techniques can allow for non-destructive surveys of large areas to pin-
point habitats frequented by soniferous species,particularly during spawning events when vocal
activity tends to be greatest. Studies of fish sounds can provide a wealth of data on temporal and
spatial distribution patterns, habitat use, and spawning, feeding,and predator avoidance behaviors.
Currently most investigators use simple omnidirectional hydrophones and can usually only locate
the general area of a spawning aggregation, but have often been forced to use circumstantial evi-
dence of the identity and behavior of the calling species (e.g.Saucer and Baltz 1993, Luczkovich et
al.1999a,b). Attempts to use passive acoustics as a tool to identify EFH based on spatial patterns in
sound production is also critically hampered by the lack of sufficient data describing the sound
characteristics of individual species and behaviors under field conditions. We propose that acoustic
technologies utilizing hydrophone arrays to home in on sound sources can greatly improve the
study of soniferous fishes and their habitat requirements. First,homing in on sound sources will
provide a valuable new tool to validate the identity of sound producers,especially when coupled
with underwater photography or video devices. Second, the ability to home in on vocal fishes
would enhance our ability to correlate fish sound production with specific locations and habitats. In
this paper,we describe our preliminary attempts to develop a Soniferous Fish Locator (SFL) for use
with a remotely operated vehicle (ROV) to home in on fish sound sources and make recommenda-
tions for future efforts.
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Proceedings from the International Workshop on the Applications of Passive Acoustics to Fisheries
Tracking and Homing Basics
The use of passive acoustics for homing was developed for naval warfare during and before WW II. A
passive acoustics homing system was implemented on torpedoes for destruction of ships and sub-
marines.The technique of homing was extended to detection and tracking of submarines by
sonobuoys during World II. These systems were developed before the advent of small, fast comput-
ers and were implemented with electronics that are now known as operational amplifiers. Adequate
Signal-to-Noise ratios were required for implementing these techniques.Homing on ships and sub-
marines by torpedoes requires only 2 directional hydrophones because the torpedo body blocks
out sound from behind it.The available aperture is small so frequencies such that there are several
wavelengths across the directional hydrophones are used for the lowest frequency in the tracking
bandwidth.The torpedo determines the bearing from acoustic signature (signal) of the ship or sub-
marine by cross-correlating the signatures from the 2 hydrophones. The cross-correlation function
is:
t+T/2
C12 (τ) = 1/T [ s1(t)s2(t+τ)] dτas T ∞(1)
t-T/2
where s1(t) and s2(t) are the noise-free time signals from the 2 hydrophones,τis the time delay
between the arrivals of the signals at their respective hydrophones and T is the period over which
the cross-correlation is estimated.The longer T is, the better the cross-correlation estimate E[C12(τ)].
For this function, there are 2 bearings or values of ( that can arise for the maximum value of
E[C12(τ)]. One represents the back direction that we know is wrong because the body of the torpe-
do blocks out that back direction. The other bearing then has to be the correct one. Hydrophone
separation, x,in homing torpedoes is small but a relatively broad segment of the noise spectra is
available to provide sufficient bearing accuracy for tracking.
The cross-correlation function of Equation (1) cannot be realized in practice but only estimated.
Accuracy of the estimate depends primarily on 1) signal-to-noise ratio (SNR), 2) bandwidth of the
signal and 3) separation of the hydrophones.Obviously for wider separations, bearing accuracy is
better.The choice of hydrophone separation is a compromise imposed by the operational require-
ments arising when one wishes to place hydrophones on an ROV. Loss of coherence depends on
environmental conditions and their effect on propagation of sound.Loss of coherence is more
severe at the higher frequencies, but it is not an important factor for arrays that will fit on an ROV.
For large signal bandwidths,the peak of the cross-correlation function is narrow.When estimating
the cross-correlation function, the time gate T imposes a (sin πf)/πf type function upon the estimate
that, along with SNR, determines how accurately one can track a fish.When the bandwidth is large
and SNR high, one can choose a small time window and/or a small hydrophone separation and get
good homing results.
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Proceedings from the International Workshop on the Applications of Passive Acoustics to Fisheries
Methods
The SFL was designed to work based on the well-understood principle of null steering on an
acoustic source with two cardiod hydrophones (Fig. 1).Specifically,the SFL consists of three
hydrophones configured to form two orthogonal cardiods shown by the solid and dotted lines (Fig.
1). The two cardiods are 180 degrees out of phase with each other in this configuration. Electronic
summing of the two cardiods results in a null along the x-axis (Fig.1). A bearing to a sound source is
obtained by rotating the SFL until the sound direction is coincident with the null. To enable an
operator to determine bearing,output from the SFL will be sent to both earphones and a recording
device. The null is found by listening to the summed output of the two cardiods in one ear while
simultaneously listening to the sound intensity with the other ear. Feeding output from cardiod 1
prior to summation with cardiod 2 to the second earphone channel eliminates noise from behind
the SFL (sound from behind the SFL is nulled out by the cardiod, Fig. 1). Hence, the operator listens
only to the intensity of sounds coming from in front of the SFL. This is an important property of the
SFL that reduces interference due to ROV noise and/or boat noise when operating in shallow water.
The distances, d, separating the hydrophones can be changed to increase or decrease the sensitivity
of the dipole and allow the operator to tune the maximum sensitivity toward the predominant fre-
quency band of the type of soniferous fish species for which he/she is searching.
Initial tests of the feasibility of deploying an array of hydrophones on a Phantom III model ROV as
part of the SFL were conducted in test tanks located at the Northeast-Great Lakes Center for the
National Under Sea Research Program at Avery Pt.Connecticut in October 2001. Test were conduct-
ed on the array configuration, attachment methods and ROV noise production. The ROV was not
able to support a hydrophone array in the required configuration (Fig. 1) because of ballast prob-
lems. We therefore had to modify the configuration so that the hydrophone array could be support-
ed by the ROV frame (Fig.2). Unfortunately,this configuration does not allow for the cancellation of
ROV noise (the array must be forward of the noise source as in Fig.1). With this configuration, noise
levels under various operating conditions were tested: 1) with all thrusters off and the ROV sitting
on the bottom, 2) with top thrusters on, 3) with rear thrusters on,and 4) with all thrusters on.
Field testing was conducted within the Stellwagen Bank National Marine Sanctuary on board the
R/V Connecticut from October 17-24, 2001. Ten ROV dives were conducted in sand,gravel and boul-
der habitats within the sanctuary. Operations were conducted in depths of up to 70 m under some-
times harsh environmental conditions and strong tidal currents.To reduce ship noise, ROV dives
were conducted while the ship was at anchor and running off of its generators. The array was com-
posed of three TH608-40 model hydrophones made by Engineering Acoustics, Inc (933 Lewis Drive,
Suite C, Winter Park, FL 32789). The hydrophones had a nominal sensitivity at the preamplifier out-
put of -160.5 dB.The 3-channel audio data from the array was captured to a laptop PC with a 4-
channel I/O board and NIDisk software supplied by Engineering Design (43 Newton St., Belmont, MA
02478). Sound signal processing was conducted using Signal 4.0 (Engineering Design). A 1 k Hz
sine wave was played through a portable CD player into the system and the input voltage recorded
at the beginning of each ROV dive. This allowed calibration of the system gain,in addition to the
hydrophone. A single channel of audio data was simultaneously recorded to video (both Hi-8 and
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Proceedings from the International Workshop on the Applications of Passive Acoustics to Fisheries
VHS) for backup. The calibration signal was also recorded to the videotape so that calibrated audio
data can be obtained directly from the tapes to obtain signal source levels.
Results
Tank tests revealed a very high level of noise, even with the thrusters turned off and the ROV sitting
motionless on the bottom of the tank (Fig.3). Although noise levels were highly variable, we esti-
mated levels of >130 dBV with thruster off and >160 dBV with all thrusters on. The high level of
noise precluded the operation of the SFL with a “flying”ROV,with the current array configuration.
We therefore decided to modify the operation of the ROV while in the field in order to increase the
signal to noise ratio enough to obtain bearing information. We required the ROV to remain station-
ary with its thrusters turned off long enough to acquire the bearing to the sound source.
With all thrusters on, the ROV produced high levels of sound at both high and low frequencies (Fig.
4). Dominant frequencies were centered on 7-8 kHz.While the stationary ROV was significantly qui-
eter,it still generated substantial noise centered on 8 kHz (Fig. 5). The low frequency noise in Fig. 5 is
an artifact resulting from mechanical banging, rubbing and tapping on the tank sides by techni-
cians testing sound reception.
Recording fish sounds in the field with an array attached to the ROV proved to be very difficult in
practice. Strong currents limited our ability to remain stationary on the bottom. The ROV was rarely
able to maintain its position on the bottom for more than a few minutes before the operator was
forced to turn on its thrusters to stabilize the vehicle. This also required the operator to turn on the
ROV lights, thus further disturbing the fishes. Fish sounds were recorded on only one occasion
when we were able to maintain the ROV on the bottom with its thrusters and lights off (Fig.6). A
prolonged series of low thumps and growls from a single fish were recorded over a 20 minute peri-
od when the ROV was sitting stationary with its lights off. During this time a large cusk, Brosme
brosme, was frequently observed hanging around the ROV. It is highly likely that the cusk is the
source of the recorded sounds. We estimated the ambient noise (ROV + ship + seas) level at around
134 dBV and the cusk call at around 140 dBV. At other times when the lights and thrusters were on,
cusk were only observed in a highly agitated state, and appeared to strongly avoid the ROV.
Discussion
Based on preliminary analysis of these data we feel that the concept of a Soniferous Locator Device
is viable. However,current ROV designs preclude optimal configuration of the hydrophone array,
requiring the SFL to be operated in a stationary mode. We propose that a vehicle specifically
designed for low noise production and capable of carrying an SFL with a 2-3 m base line in its nose
would provide an exciting new passive acoustic tool for soniferous fish surveys. The low calling rate
of the fish recorded in this study demonstrates that it would be difficult to track fish using the man-
ual null steering method proposed. Faster digital tracking using this same principal would correct
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Proceedings from the International Workshop on the Applications of Passive Acoustics to Fisheries
this problem and should be implemented in future efforts. However,it is important to point out that
data collected during this cruise demonstrates that an ROV can serve as an adequate vehicle for the
collection of underwater acoustic data even without the SFL. The ROV with a hydrophone attached
would be used to locate an optimum location and then would be set down on the bottom to record
sounds. In this way,a roving survey could be conducted.
Although cusk have long been considered to be soniferous because of the presence of a sonic mus-
cle, they had never been recorded until Norwegian scientists recently recorded their spawning
sounds (Aud Vold Soldal, Institute of Marine Research,Norway, pers. Comm.). The calls apparently
resemble haddock spawning calls and are very different from those we recorded during this study.
Our recordings were conducted well outside of the spawning season for cusk,so the sounds were
likely associated with other behavior (feeding or territorial display). Observations made subsequent
to this study revealed that cusk vigorously guard the chum bag attached to the ROV and frequently
chase away other fishes, suggesting the species is highly aggressive and territorial. Because so little
is known of the cusk’s behavior, ecology and habitat requirements, and because it appears to
respond well to a stationary ROV with its lights turned off, it makes a promising field study animal
for passive acoustics.
A secondary outcome of the cruise was that we obtained sufficient video data to suggest that the
behavior of some species is strongly influenced by the ROV and/or the ROV lights. Adult cunner,
Tautogolabrus adsperus,redfish, Sebastes fasciatus,and pollock, Pollachius virens,obviously avoided
the ROV during the day,but pollock were strongly attracted to the ROV at night due to our use of
chum and bright lights. The chum attracted swarms of amphipods that in turn attracted a large
aggregation of pollock as well as haddock, cod and skates. Cusk were only observed in boulder
habitat and avoided the mobile ROV both during the day and night when the lights were on. When
only infrared lights were used,the cusk was clearly attracted to the chum bag on the ROV and
showed no avoidance of a stationary ROV. Contrastingly, species such as cunner, redfish and silver
hake appear to avoid the ROV regardless of whether its lights are on or off, or whether it is moving
or stationary. The response of the cusk to the mobile ROV with its lights turned on suggest the
species strongly avoids the ROV. It could not be determined whether the lights or the ROV noise
caused this avoidance,however subsequent observations of cusk behavior indicate no avoidance of
stationary cameras with white lights. We suggest then, that the noise generated by the ROV can be
a significant source of bias in studies using ROVs for fish census.
Acknowledgements
We thank Meghan Hendry-Brogan for diligent work in both the field and laboratory to collect and
process fish sound data. Rebecca Jordan and David Howe assisted in the field. This project received
major funding from the Northeast and Great Lakes National Undersea Research Center,which also
provided extensive logistical support. The Woods Hole Sea Grant College Program also provided
supporting funds. The Sounds Conservancy,Quebec-Labrador Foundation/Atlantic Center for the
Environment provided a stipend for Megan’s fieldwork.
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Proceedings from the International Workshop on the Applications of Passive Acoustics to Fisheries
Literature Cited
Luczkovich, J.J., H.J.Daniel, III.,M.W.Sprague,S.E. Johnson, R.C. Pullinger,T. Jenkins,and M.
Hutchinson. 1999. Characterization of critical spawning habitats of weakfish,spotted seatrout and
red drum in Pamlico Sound using hydrophone surveys. Final Report and Annual Performance
Report Grant F-62-1 and F-62-2, Funded by the U.S. Department of the Interior, Fish and Wildlife
Service in Cooperation with the North Carolina Department of Environment and Natural Resources,
Division of Marine Fisheries, Morehead City, NC 28557. 128 p.
Luczkovich, J.J., M.W. Sprague, S.E. Johnson, and R.C. Pullinger. 1999. Delimiting spawning areas of
weakfish Cynoscion regalis (Family Sciaenidae) in Pamlico Sound,North Carolina using passive
hydroacoustic surveys. Bioacoustics 10:143-160.
Saucier,M.H., and D.M. Baltz.1993. Spawning site selection by spotted seatrout, Cynoscion nebulosus,
and black drum, Pogonias cromis, in Louisiana.Env. Biol.Fish. 36:257-272.
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Proceedings from the International Workshop on the Applications of Passive Acoustics to Fisheries
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Proceedings from the International Workshop on the Applications of Passive Acoustics to Fisheries
Illustrations and Diagrams
Y-axis
X-axis
45o
Ca rd iod 1
directivity envelop Cardiod 2
directivity envelop
Soniferous Fish Locator
H1
H2H3
45o
ROV length 1.52 m
ROV width 0 .95 m
ROV height 0. 84 m
ROV upward th rust ers,
36 cm be lo w RO V top ,
ju st fo rward of R O V cen ter ,
directed at angle intersectin g
abou t at top o f ROV pl ane
Hy dr o ph ones
Apex hydro pho ne at t o p
of ROV f rame
Bas e h ydroph one s 9 cm bel ow ap ex
Rear thru sters
Array base 1.44 m
Ar ra y si d e s 1. 4 6 m
Fo rwar d d ir ectio n of mo tion
Figure 1.Illustration of the principle of null steering on an acoustic source with two cardiod hydrophones. The
Soniferous Fish Locator consists of three hydrophones (H1-H3) configured to form two orthogonal cardiods shown by
the solid and dotted lines. The two cardiods are 180 degrees out of phase with each other. Summing the two results
in a null along the x-axis. A bearing to a sound source is obtained by rotating the SFL until the source direction is
coincident with the null.
Figure 2. Schematic illustration of the hydrophone array configuration and attachment to the Phantom III ROV.
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Proceedings from the International Workshop on the Applications of Passive Acoustics to Fisheries
Figure 3. Tank test of noise generation by the ROV with all thrusters off (lower left), top thrusters on (lower right),back
thrusters on (upper left) and all thrusters on (upper right). Digitized at 20 k Hz.
Figure 4. Noise generation from the ROV with all thrusters on. Recorded at 20 kHz.
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Proceedings from the International Workshop on the Applications of Passive Acoustics to Fisheries
Figure 5.Noise generated by the ROV while sitting on the tank bottom with all thrusters off.
Figure 6. Recording of ROV/Ship and ambient noise (bottom panel) together with the call of the cusk, Brosme
brosme. The spectrum of a 95 second sequence of multiple fish calls of a single fish is shown in the bottom panel.
The middle panels contain relative amplitude waveform,spectra and power spectra for a 5 second sequence contain-
ing only noise, while the upper panels contain a single fish call (sampled at 20 kHz and filtered above 1400 Hz).
